If someone is a native speaker of Khmer, is living in New Zealand and is learning Russian from a tutor speaking English, I'd know how to refer to Khmer (the "native language"), and how to refer to Russian (the target language). But what term would I use to refer to English in this scenario?
Should I use the term medium of instruction, or is that term more applicable to large-scale schools rather than one-on-one and class-based language learning?