Serve three languages instead of shipping ten and serving one

app/controllers/concerns/multilang.rb

@@ -3,81 +3,101 @@
 module Multilang
   extend ActiveSupport::Concern
 
+  # Kept in step with I18n.available_locales by the test below it. This used to
+  # list ten languages plus Japanese, which had no locale file and was never in
+  # available_locales at all -- the switcher would have offered a language the
+  # app could not render.
   LOCALES = {
-    de: 'Deutsch',
     en: 'English',
-    es: 'Español',
-    fa: 'الفارسية',
-    fr: 'Français',
-    it: 'Italiano',
-    ja: '日本語',
-    pl: 'Polska',
-    pt: 'Português',
     ru: 'Русский',
     zh: '中文'
-  }
+  }.freeze
 
   included do
-    # before_action :set_locale
+    around_action :set_locale
 
     helper_method :browser_locale
     helper_method :locales_for_select
-    helper_method :locale_switcher
   end
 
+  # The visitor's most-preferred language that this site can actually render.
+  #
+  # This used to be `scan(/[a-z]{2}(?=;)/)`, which only sees a tag followed by a
+  # `;` -- that is, a tag carrying a q-value. The first entry in an
+  # Accept-Language header does not carry one, so the visitor's *top* preference
+  # was the one entry the scan could never match: `ru,en;q=0.9` answered `en`.
+  # Nothing noticed, because set_locale was switched off.
   def browser_locale
-    locales = request.env['HTTP_ACCEPT_LANGUAGE'] || ''
-    locales.scan(/[a-z]{2}(?=;)/).find do |locale|
-      I18n.available_locales.include?(locale.to_sym)
-    end
+    accepted_languages.find { |tag| I18n.available_locales.include?(tag.to_sym) }
+  end
+
+  # The header's language tags, most-preferred first, minus the ones the client
+  # has ruled out. `q=0` means "not acceptable" (RFC 9110 12.4.2), so
+  # `ru;q=0,de;q=0.9` must not answer `ru` merely because `de` is not served.
+  def accepted_languages
+    entries = request.env['HTTP_ACCEPT_LANGUAGE'].to_s.split(',')
+    ranked = entries.map.with_index { |part, index| rank(part, index) }
+    ranked.reject { |_, quality, _| quality.zero? }
+          .sort_by { |entry| entry.drop(1) }
+          .map(&:first)
+  end
+
+  # One header entry, as [tag, -quality, index].
+  #
+  # The tag is cut to the two letters I18n keys on, so `de-DE` ranks as `de`.
+  # An entry with no q-value is the strongest the header carries, so it defaults
+  # to 1 rather than being skipped -- skipping it is what the old scan did.
+  # index breaks ties, because sort_by is not stable and equal q-values have to
+  # keep the order the browser sent them in.
+  def rank(part, index)
+    tag, *parameters = part.split(';')
+    [tag.to_s.strip.downcase[0, 2].to_s, -quality_of(parameters), index]
+  end
+
+  # Splitting on the literal ';q=' missed every header that spells it another
+  # way, and the grammar allows several: whitespace around the separator, and
+  # `Q` as readily as `q`. `en; q=0.1,ru;q=0.9` ranked English at 1 and picked
+  # it -- the opposite of what the visitor asked for.
+  def quality_of(parameters)
+    found = parameters.map(&:strip).find { |parameter| parameter.downcase.start_with?('q=') }
+    return 1.0 if found.nil?
+
+    found[2..].to_f
   end
 
   def self.default_url_options
     { locale: I18n.locale }
   end
 
-  def set_locale
-    # detect locale from browser languages
+  # around_action, not before_action, and I18n.with_locale rather than
+  # `I18n.locale =`. I18n.locale is per-thread and nothing resets it at the end
+  # of a request, so an assignment leaks into whatever that thread serves next.
+  # Every request does pass through here, so in practice it would be overwritten
+  # -- but "in practice" is doing the work in that sentence, and with_locale
+  # costs nothing.
+  def set_locale(&)
+    # A first visit has no choice recorded, so start from what the browser asks
+    # for. Later visits keep whatever the switcher last set.
     session[:locale] ||= browser_locale
+    session[:locale] = I18n.default_locale unless available?(session[:locale])
 
-    # save session locale value to default if not valid
-    old_locale = session[:locale].to_s.to_sym
-    unless I18n.available_locales.include?(old_locale)
-      session[:locale] = I18n.default_locale
-    end
-
-    # save session locale value from params if valid
-    if params[:locale]
-      new_locale = params[:locale].to_s.to_sym
-      if I18n.available_locales.include?(new_locale)
-        session[:locale] = new_locale
-      end
-    end
+    # The switcher links to ?locale=xx. A locale this site does not serve is
+    # ignored rather than honoured: it used to be able to leave I18n.locale set
+    # to something with no translation file behind it.
+    session[:locale] = params[:locale] if available?(params[:locale])
 
-    # set app locale from session
-    I18n.locale = session[:locale]
+    I18n.with_locale(session[:locale], &)
   end
 
-  def locales_for_select
-    I18n.available_locales.map { |l| [t("locales.#{l}"), l] }
+  def available?(locale)
+    locale.present? && I18n.available_locales.include?(locale.to_s.to_sym)
   end
 
-  def locale_switcher
-    html = []
-    html << '<ul class="navbar-nav text-uppercase">'
-    html << '<li class="nav-item dropdown">'
-    html << '<a aria-expanded="false" class="nav-link dropdown-toggle" href="#" data-bs-toggle="dropdown" id="dropsownLanguage" role="button">'
-    #html << format('%<language>s [%<locale>s]', { language: t("str.language"), locale: I18n.locale })
-    html << '<img src="/assets/translate.svg" alt="Image: language icon" class="icon img-fluid" title="Language selection">'
-    html << '</a>'
-    html << '<ul aria-labelledby="dropdownLanguage" class="dropdown-menu dropdown-menu-lg-end">'
-    I18n.available_locales.sort.each do |l|
-      html << '<li class="dropdown-item">'
-      html << format('<a href="?locale=%<locale>s" class="%<active>s">%<name>s</a>',
-                     { active: I18n.locale.eql?(l) ? ' fw-bold' : nil, locale: l, name: LOCALES[l] })
-      html << '</li>'
-    end
-    html << '</ul></li></ul>'
-    html.join("\n").html_safe
+  # LOCALES, not `t("locales.#{l}")`: there are no `locales.*` keys in any file,
+  # so every entry came back as a translation-missing span. A language name is
+  # written the same in every language anyway, which is why the switcher
+  # partial uses LOCALES too.
+  def locales_for_select
+    I18n.available_locales.map { |l| [LOCALES[l], l] }
   end
 end

app/controllers/pages_controller.rb

@@ -60,7 +60,7 @@ def our_team
   end
 
   def qr_code_generator
-    @page_title =   t('pages.qr_code.title')
+    @page_title = t('pages.qr_code_generator.title')
     render 'pages/qr_code_generator'
   end
 

app/views/layouts/_locale_switcher.html.erb

@@ -0,0 +1,34 @@
+<%#
+  Was a string-building helper in the Multilang concern. Two things it could not
+  do from there, both of which only started mattering when the switcher was
+  turned on:
+
+  - the icon was a hardcoded <img src="/assets/translate.svg">, and production
+    serves fingerprinted assets with compile off, so that is a 404 on every
+    page. icon_language has rendered it through image_tag all along.
+  - every link was a bare "?locale=xx", which replaces the whole query string.
+    Switching language on /snapshots?page=2 lost the page, and on a permanent
+    link to a camera configuration it lost the configuration.
+
+  aria-labelledby also pointed at "dropdownLanguage" while the button was
+  id="dropsownLanguage", so it labelled nothing.
+%>
+<ul class="navbar-nav text-uppercase">
+  <li class="nav-item dropdown">
+    <a aria-expanded="false" class="nav-link dropdown-toggle" href="#"
+       data-bs-toggle="dropdown" id="dropdownLanguage" role="button">
+      <%= icon_language %>
+    </a>
+    <ul aria-labelledby="dropdownLanguage" class="dropdown-menu dropdown-menu-lg-end">
+      <% Multilang::LOCALES.each do |locale, name| %>
+        <% current = I18n.locale.eql?(locale) %>
+        <li class="dropdown-item">
+          <%= link_to name,
+                      "#{request.path}?#{request.query_parameters.merge('locale' => locale).to_query}",
+                      lang: locale, class: (current ? 'fw-bold' : nil),
+                      aria: { current: (current ? 'true' : nil) } %>
+        </li>
+      <% end %>
+    </ul>
+  </li>
+</ul>

app/views/layouts/application.html.erb

@@ -47,7 +47,7 @@
           </ul>
         </li>
       </ul>
-      <!-- %= locale_switcher % -->
+      <%= render 'layouts/locale_switcher' %>
     </div>
   </div>
 </nav>

app/views/snapshots/show.html.erb

@@ -18,7 +18,7 @@
   <% if @snapshot.file.content_type.to_s.start_with?('image/hei') %>
     <p data-heif-original="<%= rails_blob_path(@snapshot.file, disposition: 'inline') %>">
       <button type="button" class="btn btn-sm btn-outline-secondary" data-heif-view>
-        <%= t('site.snapshot.view_heif', default: 'View original HEIF in your browser') %>
+        <%= t('site.snapshot.view_heif') %>
       </button>
       <span class="small text-muted ms-2" data-heif-status></span>
       <canvas class="d-none w-100 mt-2" data-heif-canvas></canvas>

config/i18n-tasks.yml

@@ -1,6 +1,6 @@
 # i18n-tasks finds and manages missing and unused translations: https://github.com/glebm/i18n-tasks
 
-locales: [en, ru, de, es, it, pl, pt, fa, zh]
+locales: [en, ru, zh]
 base_locale: en
 internal_locale: en
 

config/initializers/locale.rb

@@ -1,8 +1,21 @@
 # Where the I18n library should search for translation files
 I18n.load_path += Dir[Rails.root.join('lib', 'locale', '*.{rb,yml}')]
 
-# Permitted locales available for the application
-I18n.available_locales = [:en, :de, :es, :fr, :it, :pl, :pt, :ru, :fa, :zh]
+# Without this, an `i18n.plural.rule` in lib/locale is read and ignored, and
+# every locale gets the built-in one/other split. Russian needs four forms.
+I18n::Backend::Simple.include I18n::Backend::Pluralization
+
+# Permitted locales available for the application.
+#
+# Three, not ten. The site carried ten locale files and served none of them,
+# because Multilang's set_locale was switched off deliberately -- the project
+# did not have the people to keep ten translations in step with the English
+# copy, and a stale translation of the flashing instructions is worse than an
+# English one. Translations are cheaper to produce now; keeping them honest
+# through every later edit is not, so the list is what the project believes it
+# can maintain: English as the source, Russian and Chinese for the two largest
+# communities.
+I18n.available_locales = %i[en ru zh]
 
 # Set default locale to something other than :en
 I18n.default_locale = :en