Thank you so much. I was blind and too much focused on the language files.

Any hint with the calender-locals.js path? Do I have to switch default language with language? Whats the file that contains the path and creates the link? Links for both languages are the same. So I either have a wrong configuration ? Forum discussed this issue before, and still I cannot find a solution.

...plugin/language/default-language/calender-locals.js

Thats true the API has a lot of information.
To get back to the language that gets picked in the form:

I do see that there was an open ticket with the calendar-locals.js
The path to this js file just seems to be the default-language folder in either language.

www.joomlapolis.com/forum/180-cb/158799-solved-ticketforge-cb-14-kalender-english

Is there any new outcome as of yet?

Thanks again, Nick I do aprreciate your help - I am still a newbie here

@ strawberry

the file is
...\administrator\components\com_comprofiler\ue_config_first.php and ue_config.php

Have you found a way to translate it?
so that every body gets the e-mail inthe language they registered on?

The email confirmations are sent out in the correct language. This is through the "language variables".
I also found a solution for the calender-locals.js.
The language folder name needs to be de-DE and not de-de, then javascript gives out the correct path and finds its file.
Unfortunately if I delete the "old" folder de-de my translation files are not getting found anymore and I wont get a translation. So for now I will need to produce "double content". ONe in the de-de folder for the language.php and the one de-DE for the javascript-file.