Tune of Translation

For the duration, all creatures that speak a language understand what you are saying as if you were speaking in their language. 

This does not give the caster the ability to comprehend a language he does not know that is spoken in return.

The caster must be playing an instrument that they are proficient in while they are speaking. No proficiency check required.

This applies only to spoken language and does not confer the ability to write in any language not known.