Use browser Accept-Language for login and onboarding locale (#768)

* Use Accept-Language for unauthenticated locale

* Add per-user locale overrides

* Fix test

* Use more than the top `accept-language` entry

* Localization of string

app/controllers/concerns/localize.rb

@@ -8,12 +8,104 @@ module Localize
 
   private
     def switch_locale(&action)
-      locale = locale_from_param || Current.family.try(:locale) || I18n.default_locale
+      locale = locale_from_param || locale_from_user || locale_from_accept_language || locale_from_family || I18n.default_locale
       I18n.with_locale(locale, &action)
     end
 
+    def locale_from_user
+      locale = Current.user&.locale
+      return if locale.blank?
+
+      locale_sym = locale.to_sym
+      locale_sym if I18n.available_locales.include?(locale_sym)
+    end
+
+    def locale_from_family
+      locale = Current.family&.locale
+      return if locale.blank?
+
+      locale_sym = locale.to_sym
+      locale_sym if I18n.available_locales.include?(locale_sym)
+    end
+
+    def locale_from_accept_language
+      locale = accept_language_top_locale
+      return if locale.blank?
+
+      locale_sym = locale.to_sym
+      return unless I18n.available_locales.include?(locale_sym)
+
+      # Auto-save detected locale to user profile (once per user, not per session)
+      if Current.user.present? && Current.user.locale.blank?
+        Current.user.update_column(:locale, locale_sym.to_s)
+      end
+
+      locale_sym
+    end
+
+    def accept_language_top_locale
+      header = request.get_header("HTTP_ACCEPT_LANGUAGE")
+      return if header.blank?
+
+      # Parse language;q pairs and sort by q-value (descending), preserving header order for ties
+      parsed_languages = parse_accept_language(header)
+      return if parsed_languages.empty?
+
+      # Find first supported locale by q-value priority
+      parsed_languages.each do |lang, _q|
+        normalized = normalize_locale(lang)
+        canonical = supported_locales[normalized.downcase]
+        return canonical if canonical.present?
+
+        primary_language = normalized.split("-").first
+        primary_match = supported_locales[primary_language.downcase]
+        return primary_match if primary_match.present?
+      end
+
+      nil
+    end
+
+    def parse_accept_language(header)
+      entries = []
+
+      header.split(",").each_with_index do |entry, index|
+        parts = entry.split(";")
+        language = parts.first.to_s.strip
+        next if language.blank?
+
+        # Extract q-value, default to 1.0
+        q_value = 1.0
+        parts[1..].each do |param|
+          param = param.strip
+          if param.start_with?("q=")
+            q_str = param[2..]
+            q_value = Float(q_str) rescue 1.0
+            q_value = q_value.clamp(0.0, 1.0)
+            break
+          end
+        end
+
+        entries << [ language, q_value, index ]
+      end
+
+      # Sort by q-value descending, then by original header order ascending
+      entries.sort_by { |_lang, q, idx| [ -q, idx ] }.map { |lang, q, _idx| [ lang, q ] }
+    end
+
+    def supported_locales
+      @supported_locales ||= LanguagesHelper::SUPPORTED_LOCALES.each_with_object({}) do |locale, locales|
+        normalized = normalize_locale(locale)
+        locales[normalized.downcase] = normalized
+      end
+    end
+
+    def normalize_locale(locale)
+      locale.to_s.strip.gsub("_", "-")
+    end
+
     def locale_from_param
       return unless params[:locale].is_a?(String) && params[:locale].present?
+
       locale = params[:locale].to_sym
       locale if I18n.available_locales.include?(locale)
     end

app/controllers/users_controller.rb

@@ -105,8 +105,8 @@ class UsersController < ApplicationController
     def user_params
       params.require(:user).permit(
         :first_name, :last_name, :email, :profile_image, :redirect_to, :delete_profile_image, :onboarded_at,
-        :show_sidebar, :default_period, :default_account_order, :show_ai_sidebar, :ai_enabled, :theme, :set_onboarding_preferences_at, :set_onboarding_goals_at,
-        family_attributes: [ :name, :currency, :country, :locale, :date_format, :timezone, :id ],
+        :show_sidebar, :default_period, :default_account_order, :show_ai_sidebar, :ai_enabled, :theme, :set_onboarding_preferences_at, :set_onboarding_goals_at, :locale,
+        family_attributes: [ :name, :currency, :country, :date_format, :timezone, :locale, :id ],
         goals: []
       )
     end

app/models/user.rb

@@ -39,6 +39,7 @@ class User < ApplicationRecord
   validate :ensure_valid_profile_image
   validates :default_period, inclusion: { in: Period::PERIODS.keys }
   validates :default_account_order, inclusion: { in: AccountOrder::ORDERS.keys }
+  validates :locale, inclusion: { in: I18n.available_locales.map(&:to_s) }, allow_nil: true
 
   # Password is required on create unless the user is being created via SSO JIT.
   # SSO JIT users have password_digest = nil and authenticate via OIDC only.

app/views/onboardings/preferences.html.erb

@@ -76,7 +76,7 @@
 
           <%= family_form.select :locale,
             language_options,
-            { label: t(".locale"), required: true, selected: params[:locale] || @user.family.locale },
+            { label: t(".locale"), required: true, selected: params[:locale] || @user.locale || I18n.locale },
             { data: { action: "onboarding#setLocale" } } %>
 
           <%= family_form.select :currency,

app/views/settings/preferences/show.html.erb

@@ -5,16 +5,16 @@
     <%= styled_form_with model: @user, class: "space-y-4", data: { controller: "auto-submit-form" } do |form| %>
       <%= form.hidden_field :redirect_to, value: "preferences" %>
 
+      <%= form.select :locale,
+          language_options,
+          { label: t(".language"), include_blank: t(".language_auto") },
+          { data: { auto_submit_form_target: "auto" } } %>
+
       <%= form.fields_for :family do |family_form| %>
         <%= family_form.select :currency,
             Money::Currency.as_options.map { |currency| [ "#{currency.name} (#{currency.iso_code})", currency.iso_code ] },
             { label: t(".currency") }, disabled: true %>
 
-        <%= family_form.select :locale,
-            language_options,
-            { label: t(".language") },
-            { data: { auto_submit_form_target: "auto" } } %>
-
         <%= family_form.select :timezone,
             timezone_options,
             { label: t(".timezone") },

config/locales/views/settings/ca.yml

@@ -41,6 +41,7 @@ ca:
         general_subtitle: Configura les teves preferències
         general_title: General
         language: Idioma
+        language_auto: Idioma del navegador
         page_title: Preferències
         theme_dark: Fosc
         theme_light: Clar

config/locales/views/settings/de.yml

@@ -34,6 +34,7 @@ de:
         default_period: Standardzeitraum
         default_account_order: Standardreihenfolge der Konten
         language: Sprache
+        language_auto: Browsersprache
         page_title: Einstellungen
         theme_dark: Dunkel
         theme_light: Hell

config/locales/views/settings/en.yml

@@ -36,6 +36,7 @@ en:
         default_period: Default Period
         default_account_order: Default Account Order
         language: Language
+        language_auto: Browser language
         page_title: Preferences
         theme_dark: Dark
         theme_light: Light

config/locales/views/settings/es.yml

@@ -35,6 +35,7 @@ es:
         default_period: Periodo Predeterminado
         default_account_order: Orden Predeterminado de Cuentas
         language: Idioma
+        language_auto: Idioma del navegador
         page_title: Preferencias
         theme_dark: Oscuro
         theme_light: Claro

config/locales/views/settings/fr.yml

@@ -35,6 +35,7 @@ fr:
         default_period: Période par défaut
         default_account_order: Ordre d'affichage des comptes par défaut
         language: Langue
+        language_auto: Langue du navigateur
         page_title: Préférences
         theme_dark: Sombre
         theme_light: Clair

config/locales/views/settings/nb.yml

@@ -19,6 +19,7 @@ nb:
         general_title: Generelt
         default_period: Standardperiode
         language: Språk
+        language_auto: Nettleserens språk
         page_title: Preferanser
         theme_dark: Mørk
         theme_light: Lys

config/locales/views/settings/nl.yml

@@ -35,6 +35,7 @@ nl:
         default_period: Standaard Periode
         default_account_order: Standaard Account Volgorde
         language: Taal
+        language_auto: Browsertaal
         page_title: Voorkeuren
         theme_dark: Donker
         theme_light: Licht

config/locales/views/settings/pt-BR.yml

@@ -34,6 +34,7 @@ pt-BR:
         general_title: Geral
         default_period: Período Padrão
         language: Idioma
+        language_auto: Idioma do navegador
         page_title: Preferências
         theme_dark: Escuro
         theme_light: Claro

config/locales/views/settings/ro.yml

@@ -35,6 +35,7 @@ ro:
         default_period: Perioadă implicită
         default_account_order: Ordine implicită conturi
         language: Limbă
+        language_auto: Limba browserului
         page_title: Preferințe
         theme_dark: Întunecat
         theme_light: Luminos

config/locales/views/settings/tr.yml

@@ -19,6 +19,7 @@ tr:
         general_title: Genel
         default_period: Varsayılan Dönem
         language: Dil
+        language_auto: Tarayıcı dili
         page_title: Tercihler
         theme_dark: Koyu
         theme_light: Açık

config/locales/views/settings/zh-CN.yml

@@ -38,6 +38,7 @@ zh-CN:
         general_subtitle: 配置个人偏好设置
         general_title: 通用设置
         language: 语言
+        language_auto: 浏览器语言
         page_title: 偏好设置
         theme_dark: 深色模式
         theme_light: 浅色模式

config/locales/views/settings/zh-TW.yml

@@ -35,6 +35,7 @@ zh-TW:
         default_period: 預設時段
         default_account_order: 預設帳戶排序
         language: 語言
+        language_auto: 瀏覽器語言
         page_title: 偏好設定
         theme_dark: 深色
         theme_light: 淺色

db/migrate/20260124180211_add_locale_to_users.rb

@@ -0,0 +1,6 @@
+class AddLocaleToUsers < ActiveRecord::Migration[7.2]
+  def change
+    add_column :users, :locale, :string, null: true, default: nil
+    add_index :users, :locale
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_01_23_214127) do
+ActiveRecord::Schema[7.2].define(version: 2026_01_24_180211) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -1402,9 +1402,11 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_23_214127) do
     t.datetime "set_onboarding_goals_at"
     t.string "default_account_order", default: "name_asc"
     t.jsonb "preferences", default: {}, null: false
+    t.string "locale"
     t.index ["email"], name: "index_users_on_email", unique: true
     t.index ["family_id"], name: "index_users_on_family_id"
     t.index ["last_viewed_chat_id"], name: "index_users_on_last_viewed_chat_id"
+    t.index ["locale"], name: "index_users_on_locale"
     t.index ["otp_secret"], name: "index_users_on_otp_secret", unique: true, where: "(otp_secret IS NOT NULL)"
     t.index ["preferences"], name: "index_users_on_preferences", using: :gin
   end

test/controllers/concerns/localize_test.rb

@@ -1,23 +1,55 @@
 require "test_helper"
 
 class LocalizeTest < ActionDispatch::IntegrationTest
-  setup do
-    sign_in users(:family_admin)
+  test "uses Accept-Language top locale on login when supported" do
+    get new_session_url, headers: { "Accept-Language" => "fr-CA,fr;q=0.9" }
+    assert_response :success
+    assert_select "button", text: /Se connecter/i
   end
 
-  test "uses family locale by default" do
-    get preferences_onboarding_url
+  test "falls back to English when Accept-Language is unsupported" do
+    get new_session_url, headers: { "Accept-Language" => "ru-RU,ru;q=0.9" }
+    assert_response :success
+    assert_select "button", text: /Log in/i
+  end
+
+  test "uses Accept-Language for onboarding when user locale is not set" do
+    sign_in users(:family_admin)
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "es-ES,es;q=0.9" }
+    assert_response :success
+    assert_select "h1", text: /Configura tus preferencias/i
+  end
+
+  test "falls back to family locale when Accept-Language is unsupported" do
+    sign_in users(:family_admin)
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "ru-RU,ru;q=0.9" }
     assert_response :success
     assert_select "h1", text: /Configure your preferences/i
   end
 
+  test "respects user locale override even when Accept-Language differs" do
+    user = users(:family_admin)
+    user.update!(locale: "fr")
+    sign_in user
+
+    get preferences_onboarding_url, headers: { "Accept-Language" => "es-ES,es;q=0.9" }
+    assert_response :success
+    assert_select "h1", text: /Configurez vos préférences/i
+  end
+
   test "switches locale when locale param is provided" do
+    sign_in users(:family_admin)
+
     get preferences_onboarding_url(locale: "fr")
     assert_response :success
     assert_select "h1", text: /Configurez vos préférences/i
   end
 
   test "ignores invalid locale param and uses family locale" do
+    sign_in users(:family_admin)
+
     get preferences_onboarding_url(locale: "invalid_locale")
     assert_response :success
     assert_select "h1", text: /Configure your preferences/i

test/controllers/users_controller_test.rb

@@ -21,14 +21,15 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
           name: "New Family Name",
           country: "US",
           date_format: "%m/%d/%Y",
-          currency: "USD",
-          locale: "en"
-        }
+          currency: "USD"
+        },
+        locale: "es"
       }
     }
 
     assert_redirected_to settings_profile_url
     assert_equal "Your profile has been updated.", flash[:notice]
+    assert_equal "es", @user.reload.locale
   end
 
   test "admin can reset family data" do