Translations were stored in a $GLOBALS array and handled by a static class Piwik\Translate. Furthermore, handling of translations were spread all over the place, caching logic was mixed with translation logic, …

This refactoring contains:

• created a Translation component following the naming of Symfony's Translation component ("Translator" class, "Loaders", …)
• all logic related to loading/handling translations is now encapsulated in that component
• classes of that component are using dependency injection
• Piwik\Translate is kept for BC and forwards call to the new Translator class
• that new component is decoupled from the rest of Piwik (except for a few exceptions to improve later): it doesn't know about the HTTP request, about Plugins, about Piwik directories, etc...
• translations are lazily loaded, i.e. you ask for a french translation then french translations will be loaded automatically
• that means that Piwik/plugins don't have to tell the translator to load anything: the plugin manager just configures the directories where to find translations (which are the installed plugins…)
• there is a LoaderInterface with the following implementations:
  • JsonFileLoader
  • LoaderCache: caches another loader by wrapping it (which would be the JsonFileLoader)
• that way of caching things brings the following benefits:
  • caching can be configured in DI, and disabled with the container using the new dev environment (which addresses this TODO)
  • core translations are cached as well as the plugin ones, which fixes Improve performance by caching all translations in a flat array #6060
• as said above, there's a new dev environment (= new DI config file) where translation cache is disabled: this environment can be used for other things as well
• there is now a fallback mechanism (previously all translations were merged together in one array): if a french/… translation is not found, then the english one is returned
• that fallback mechanism would allow to do Add support for a Translation file to extend another language (eg. Spanish Argentina extends spanish) #4983 (regional overrides) by having multiple fallbacks, for example: fr_CA (canadian french) would fallback to fr which would fallback to en (not implemented)
• moved Piwik\Translate\Writer and its subclasses to the LanguagesManager plugin because these classes were only used in that plugin

And I was able to unit tests the new classes, which is just an awesome feeling :)

There's still a lot of room for improvement, I'll keep working on that PR. Feedback welcome!