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