Introduce event language detection

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

priv/repo/migrations/20210819161507_add_language_to_entities.exs

@@ -0,0 +1,17 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddLanguageToEntities do
+  use Ecto.Migration
+
+  def change do
+    alter table(:events) do
+      add(:language, :string, default: "und")
+    end
+
+    alter table(:comments) do
+      add(:language, :string, default: "und")
+    end
+
+    alter table(:posts) do
+      add(:language, :string, default: "und")
+    end
+  end
+end