Use i18n

Description

Users should be able to choose their language in settings, which changes the language for interface and data which app is using/showing.

Explanation

Because a part of the users of the app are speaking languages other than German.

• use translations instead of hard-coded words in data templates (added to be solved separately from !2 (merged) )
• translate context menu items (The bucket keys should be translated (if available) #65 (closed))

Mockups/Screenshots

Not available.

Dependencies, issues to be resolved beforehand

i18n should be modeled in a functional way in core#10 (closed).

Edit: related to #65 (closed), please see and make use of commits from !35 (closed): 1395e49a and cde9d699 (which need to be improved).