Fallback to a variant if the language in general is not supported

For an example, here, zh is not supported, but zh_Hans and zh_Hant
are. If the user asks for zh, we should choose a variant for them
instead of fallbacking to default.

Some browsers (e.g. Firefox) does not allow users to customize
their language codes. For example, there is no zh-Hans, but only
zh, zh-CN, zh-TW, zh-HK, etc. This provides a workaround for
those users suffering from bad design decisions.

lib/pleroma/web/gettext.ex

@@ -49,6 +49,20 @@ def supports_locale?(locale) do
     |> Enum.member?(locale)
   end
 
+  def variant?(locale), do: String.contains?(locale, "_")
+
+  def supported_variants_of_locale(locale) do
+    cond do
+      variant?(locale) ->
+        [locale]
+      supports_locale?(locale) ->
+        [locale]
+      true ->
+        Gettext.known_locales(Pleroma.Web.Gettext)
+        |> Enum.filter(fn l -> String.starts_with?(l, locale <> "_") end)
+    end
+  end
+
   def locale_or_default(locale) do
     if supports_locale?(locale) do
       locale

lib/pleroma/web/plugs/set_locale_plug.ex

@@ -20,6 +20,12 @@ defp get_locale_from_header(conn) do
     conn
     |> extract_preferred_language()
     |> normalize_language_codes()
+    |> first_supported()
+  end
+
+  defp first_supported(locales) do
+    locales
+    |> Enum.flat_map(&Pleroma.Web.Gettext.supported_variants_of_locale/1)
     |> Enum.find(&supported_locale?/1)
   end
 

test/pleroma/web/plugs/set_locale_plug_test.exs

@@ -47,6 +47,20 @@ test "use supported locale with specifiers from `accept-language`" do
     assert %{locale: "zh_Hans"} == conn.assigns
   end
 
+  test "fallback to some variant of the language if the unqualified language is not supported" do
+    conn =
+      :get
+      |> conn("/cofe")
+      |> Conn.put_req_header(
+        "accept-language",
+        "zh;q=0.9, en;q=0.8, *;q=0.5"
+      )
+      |> SetLocalePlug.call([])
+
+    assert "zh_" <> _ = Gettext.get_locale()
+    assert %{locale: "zh_" <> _} = conn.assigns
+  end
+
   test "use supported locale from cookie" do
     conn =
       :get