For English speakers:
- LOTE : Language other than English
- Second language. note: as stevec has listed also here
- Additional language
- Foreign language
- Language being learned
- Target language
While often referred to as the " mother tongue / language you know / first language / known language " these terms may be called "source language" in this case.
- L2
in this case L2 is standing for "Language 2" or "additional language". Where this terminology is used " mother tongue / language you know / first language / known language " is usually referenced by L1.
This terminology is also useful if you are wishing to refer to further languages being learned/known/referred to. ie. "They are learning L2 through their knowledge of L3." Where L3 is another language to L1 (first language) that they have already learned.