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/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") },