Term for second language learned?

Question

I used 'mother tongue' to describe in conversation my first language.

Are there any terms that are used formally or informally to specifically refer to someone's second language?

Answer 1

Second language is a good term.

I have often seen target language too to speak about the language you are currently learning, but only when you're speaking about learning the language.

Answer 2

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.