Add i18n support

[mpoehler]

Add it in the angular app, but what about the backend?

[mpoehler]

The detection of the users location can be done using appengine headers like in this example: http://mylocationtest.appspot.com/

But also the preferred language from the browser (ServletRequest.getLocale) should be taken into account.

For logged in users, the selected language should overwrite everything else.

[mpoehler]

Maybe it is a good idea to place a Location / Language instance in the session scope

[mpoehler]

The solution for angular is: https://github.com/angular-translate/angular-translate

[mpoehler]

Some decisions, after some reading:

For authenticated users we need a setting where we set the language/country settings for each user. We can init it with the browser locale when the user logs in the first time. That locale will be stored as property of the user and will affect the language of emails and used as initial locale when the user logs in again. For not authenticated users we detect the browser locale and use it for i18n.