I'd like to get a sense of how likely it is that there will be a desire to have targeted alternative text for audio and video, possibly for the uses you mentioned. If you expect it's likely, I think it's worth spending a bit more time trying to come up with a way to express it in XLSForm that doesn't require this explosion of columns. It could be something like a single alt_text::lang column with keys/value pairs like we do with parameters, for example (image="", audio=""). In that case we'd want to take that approach in XLSForm from the start.
Unless anyone else has anything to add I think the client work could start any time introducing new forms.