Multiple fixes and improvements for the german translation

Oh hai

While testing the new 1.0.0 release I found a bunch of typos, missing strings and completely missing translations (e.g. in guacamole-auth-totp and in guacamole-auth-cas) in the german language files.

Is someone already working on this? If not, do I just need to update guacamole-client/guacamole/src/main/webapp/translations/de.json or are there any other files?

For the translation of the extension, can I just copy guacamole-client/extensions/guacamole-auth-*/src/main/resources/translations/en.json to de.json and translate it?

This is my first time trying to get involved in open source, please contact me if I did something wrong in the process.

best regards