fixes #7926

fixes #7736 While loading Piwik, the top menu displays all language names in a row for 1 second and then hide it

refs #7567 - use transifex statistics api and git modification time to determine which resources needs to be updated

Fix two regressions where language would not change in installer, and where language would reset to english in the middle of the install process.

Problem was it did load the language while the user was not authenticated
yet. We need to detect the language again after the user was authenticated.

fixes https://github.com/piwik/piwik/issues/7184

This language is shown when development mode is enabled.

These classes are only used in that plugin, so it doesn't make sense to keep then in Core.

Translations are now lazily loaded instead of being explicitely loaded by other components (e.g. the front controller, the plugin manager, etc...). Now other parties only configure "directories" where the translator can find translations. That makes the translator decoupled from the rest of Piwik.