Handle missing translations

[Splines]

In #749, I've set:

config.i18n.raise_on_missing_translations = true

for our development environment. The thing is that you only get these exceptions if you navigate to a respective view that actually contains a Ruby statement pointing to a missing translation. We would likely want to catch all missing translations in the end.

In the production environment, we should definitely not activate this option since otherwise users will be very frustrated when key functionality is not working anymore just because we have a missing translation. However, these cases shouldn't go totally unnoticed, so:

• we should check whether these cases already trigger a warning in the console in the production environment
• we could implement a custom missing translation handler and for our own usage log the missing translation, e.g. by sending a mail as usual error messages do. This way we could treat these cases without having to actually raise errors to the users. I.e. a kind of warning mechanism.