Use weblate for managing UI translations

[vzahradnik]

Is your feature request related to a problem? Please describe.
The way TcMenu app and plugin UI localization works is non-systematic. It is hard to track changes in English strings as the changes occurr in regular pull requests. Also, the barrier for the translators is higher as they have to be familiar with git, github, creating pull requests, etc.

Describe the solution you'd like
All the translations should be managed through the professional tools with which we will integrate our libraries. Weblate seems to be our best bet as it's completely open-source but likely other tools like Transifex are also free to use for the open-source projects like ours. We will have to do some research.

[vzahradnik]

There seems to be a free weblate option for open-source projects at https://hosted.weblate.org/