Fix translation, fix reinstall table already exists, fix deprecated datetime, Add german translation

[invisiblemarcel]

These changes are pulled separately because of the major changes.

• I always needed to drop the tables manually when I uninstalled the plugin, so I added 2 checks to avoid that.
• It was not enough to copy a .po File for translation, I needed to compile them so I add a note in the Readme and compiled all lang files and added them to the repo (I know binary files in git are crap but it's much easier for the users)
• German translation added
• The table column type datetime is deprecated so I changed it to timestamp, before you always needed to run a command displayed in the UI to migrate it

[edgardmessias]

Please, could you rebase your PR?
My suggestion: split the PR for each type of change