add translation logic, locales for en and de

This adds a basic "replace fixed keys"-approach to translating the UI of
surveys. So far this works great, but for complex things it may fail for
some languages — for now I'll just  try to take care not to add any
output that puts things in the middle of sentences, which I think should
work for a simple survey thing.

The default language is still english, which is important as the
language is declared withing a survey config — i.e. if loading it fails,
it cannot be applied, and the "enter passphrase" dialog can also not
change based on locales. This could possibly be fixed by adding an
unencrypted "header" to the encrypted configs, but for now I'm not sure
if that makes a lot of sense — it may be easier to just guess based on
the browser's settings, and otherwise fall back to some specified
default language.

The "thanks for your answers"-page also has no translation so far.

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