Set PHP locale for date format and month names

[kevinpapst]

Add subscriber that sets the locale by the current users language:
setlocale(LC_ALL, $locale1, $locale2);

Testing:

• Invoices: the "timesheet" renderer displays a month name, which must be translated properly
• Timesheets: the rate must be formatted correctly

[kevinpapst]

setlocale is not thread safe, so we do it in the view directly instead

[kevinpapst]

The rate formatting is now managed with twig filter and language specific

[lock]

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