Was muss ich schreiben?

It’s best to write about how good you speak German.

If you’re talking to native speakers, maybe I’d write “2nd mother tongue.” Otherwise, “safe for negotiation” is always a good name for it if you can interpret and apply fine nuances of a foreign language. If all this is not true, simply take the schoolnote system or hold the language level system according to the European reference framework (A1, C2, …)