This is the worry that stops people walking in to me, and it is the one that survives contact with reality least well.
Most of the instruction is not verbal
A teacher demonstrates. You copy. That is the mechanism, and it is the same mechanism in every language. The words are running alongside as commentary and correction, and you will pick up the twenty or so that recur within two classes.
Arriba. Abajo. Respira. That is a functioning vocabulary.
What you might miss
Timing cues, mostly, and the occasional joke. Missing a joke in a yoga class is survivable.
The one thing worth not missing is anything about an injury or a limitation, and that is a conversation to have before the class starts rather than during it. Write it on your phone in advance if you need to. A teacher who knows about your shoulder will find a way to communicate around it for an hour.
Say something first
Arrive a few minutes early and tell me what you need. Not the whole story, just the useful part. Two sentences. It is the single most valuable thing you can do for me, and it works in either language or in both at once, which is how most conversations here happen anyway.
The town
Sayulita is bilingual in an unforced way. Plenty of people switch mid-sentence and nobody is keeping score. Attempting Spanish badly is not an imposition here, it is normal behaviour, and the response is usually warmer than the attempt deserved.
If you are staying more than a week, learn the numbers and the days. That alone gets you through booking a class, buying almost anything and reading a timetable, and it takes an evening.
The practical version
Message me ahead if it matters to you. Turn up early. Copy what you see. You will be fine, and the class you were nervous about will turn out to be the least complicated part of your day.