Fix extraction of locale from translation files

The `LocaleExtractor` recognizes these kind of files:

  - `<locale>.yaml`
  - `<locale>_something.yaml`

It does not recognize these files we use:

  - `js-<locale>.yaml` (locale is extracted as `js-<locale>`)
  - `<locale>.seeders.yaml` (locale is extracted as `<locale>.seeders`)

It's fixed by deleting the `js-` prefix and the `.seeders` suffix from
the extracted locale.

spec/support/i18n_lazy_loading.rb

@@ -39,7 +39,7 @@ class I18n::Backend::LocaleExtractor
 
     return if locale.nil?
 
-    locale.tr("js-", "").to_sym
+    locale.delete_prefix("js-").delete_suffix(".seeders").to_sym
   end
 end