Save user locale and use it to translate things

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/mobilizon_web/gettext.ex

@@ -21,4 +21,27 @@ defmodule MobilizonWeb.Gettext do
   See the [Gettext Docs](https://hexdocs.pm/gettext) for detailed usage.
   """
   use Gettext, otp_app: :mobilizon
+
+  def put_locale(locale) do
+    locale = determine_best_locale(locale)
+    Gettext.put_locale(MobilizonWeb.Gettext, locale)
+  end
+
+  @spec determine_best_locale(String.t()) :: String.t()
+  def determine_best_locale(locale) do
+    locale = String.trim(locale)
+    locales = Gettext.known_locales(MobilizonWeb.Gettext)
+
+    cond do
+      # Either it matches directly, eg: "en" => "en", "fr" => "fr", "fr_FR" => "fr_FR"
+      locale in locales -> locale
+      # Either the first part matches, "fr_CA" => "fr"
+      split_locale(locale) in locales -> split_locale(locale)
+      # Otherwise default to english
+      true -> "en"
+    end
+  end
+
+  # Keep only the first part of the locale
+  defp split_locale(locale), do: locale |> String.split("_", trim: true, parts: 2) |> hd
 end