Fallback language

[brunnopleffken]

If a translation key doesn't exist in the selected dictionary, i18n module should fallback to default: English (US). I still don't know if should I let administrator disable this feature, because if it's disabled the user will see something like E_LOGIN_ERROR in place.

• Remove __() from non-translatable strings (e.g. results from databases or user input data) to reduce server/database over processing;
• Implement fallback script in \AC\Kernel\i18n;
• When language key is missing, it should be registered in admin logs;
• Administrator can enable or disable this feature (???).