Honestly I`m not quite sure how to describe it. The first few phrases can be a mix of two languages, and then it somehow settles into one both speakers are comfortable with. Something like...

- Мне пожалуйста, хот-дог и Кока-колу.
- Tās būs lats un deviņdesmit pieci... (Это будет один лат девяносто пять)
Siņepes, kečups?

After that I usually reply in Latvian and it continues like that from there, but if I'm distreacted,I can say "с горчицей" or, worse, make some gross error like "ar sinepes, lūdzu" (instead of the correct one, "ar siņepiem." Or was that the correct one? Why Latvians decided that mustard has to be always-plural? ) I really don't speak different languages that well unless I had an hour or two to "shift gears" so to speak... Then the seller can reply in Russian or she won't, and I'd have to correct myself... Or it may continue to be "bilingual".

Sort of like that. That's how we do it In Riga while doing important business, like selling something.

Somewhere in rural southeast Latvia it could be some local jargon no one else speaks (Ko tu броду по manu огороду? Ceļa рядом nau?) Or if the place is more monoethnic, you WILL get funny looks and jeers for speaking anything but local Latvian (or Russian or Latgalian )...

Sorry for any bad English...