thank you for your answer. I'm going to be a little painful with this one.
Unfortunately working with one xlsx for all languages (20+ with several sublanguages) is not an option as the number of source terms differs in many cases.
I have installed Multiterm 2017 SR1 18.104.22.168908, and here does Glossary Converter not support sdltb as output.
To make things even more weird, I'm currently testing on a VM Multiterm 2019 22.214.171.12489 where Glossary Converter works fine for sdltb-output.
I found this on the matter after doing some digging:
So, after this I made the good old xlsx->xml conversion with Glossary Converter, and imported these xml's into the termbase set up with several sublanguages. Unfortunately, the problem still remains, I import for Swiss German, everything is set up for Swiss German and it overwrites the Swiss entries to Austrian and leaves Swiss entries empty. I have not selected anything for Austrian, altough it the termbase contains this sublanguage, too - this should not be a problem for a multilingual database.
As you can see, the XML is set up properly for DE-CH...
but after importing it becomes Austrian.
Maybe I should try to write some rules in Excel to get every entry for all languages into one sheet which I wanted to avoid as the source terms differ in many cases.