Translation:Excuse me, do you speak either French or English?
I disagree. If you need to find someone who speaks at least one of those languages (for communicating, etc.), it does not matter if they also speak the other. If I speak both French and English, will you refuse to talk to me? :)
I would think "tai" is correct just as they have it.
Then you would not use the continuous, which implies the speaker is speaking a specific language at this very moment. The asker is not asking about somebody’s ability to speak more than one language, but trying to identify which language the speaker is speaking at this moment. This is exclusive, even if the speaker is indeed able to speak both French and English.
that Finnish has no continuous tense.
Correct. But languages that do not use any verbal form to express continuity of action have other means of conveying this concept.
Because the original Finnish sentence uses ”tai”, it is asking whether the other person is generally able to speak language A and/or B. If the asker wanted to know what language the other person is speaking at this very moment (A or B) would require ”vai”.
As a native English speaker, I'd say that "Either A or B" in English isn't automatically exclusive. English doesn't really make that distinction here without extra words like "but not both". If you ask me whether I speak either English or French, and I happen to speak both languages, I will truthfully answer "yes".
The idea, as I understand it, is that either "tai" or "vai" can be used in the Finnish question, with the same English translation but with different meanings.
It's the partitive case. Ranska -> ranskaa, englanti -> englantia and so on.
With languages the partitive is used for example in the context of speaking, knowing or understanding the language. With the idea being along the line that the action is never about the whole language, only a part.
Whereas if for example describing the language, partitive is not used. Ranska on vaikea kieli (French is a difficult language).