Implement localization/i18n support

We agreed that it would be nice to have the client localized, but currently the strings visible to the user are hard-coded into the QT .ui files and Python files. Therefore the code needs to be changed to enable localization. I suggest the following:

- [ ] Describe the current implementation regarding string distribution/handling.
- [ ] Gather requirements how these strings shall be made translatable.
- [ ] List/evaluate possible solutions (e. g. tools for string extraction) how to achieve the goal.
- [ ] Implement the chosen solution.

In Java it is possible to add a translation to a program on the fly because for each language a plain text file (.properties) is used to create a message key -> translation mapping.