One of Gemini's explicit goals is for client implementation to be so simple as to be a weekend project for the average developer.
That's one reason inline links are not allowed (they have to be a separate line): it would complicate the parser.
It shouldn't be too hard to add a "language switch" line, so that sections (but not words within sections) can be different languages.
Making Wiktionary or Etymonline in Gemini would require you to add a line break for each foreign word, which seems annoying to read.
I don't see though, why the spec can't allow links that are written on a newline for markup simplicity but displayed inline? And then the same could be done for foreign words.
You could still do multi-lingual content just by having it in separate .gmi pages and then linking between then. And your screenreader could follow links by embedding the text in the other language in the first page.
I agree with you that those behind Gemini should take this issue seriously, but maybe something like what I've mentioned is what they have in mind as their answer. Not sure.
A text only web is pretty niche. Can this project really afford to start slicing off additional portions of its userbase? I agree that multilingual blind users are in the minority, but those users are probably the most likely of anyone to be interested in a text-only web.
And even ignoring blind users, it's just... what's the point of any of this if Gemini is not going to be semantic? It's like religiously adhering to the GNU principles of always printing human-readable text on the command line, and then not shipping a shell with a pipe command or any text parsing capabilities. From just about any perspective, whether you're worried about accessibility, or parsing, or search -- it's useful to know what language a document/section is in.
One of the few advantages of having a limited, well-defined, non-extensible spec is that it's easy to work with and write parsers around. And immediately Gemini is just saying, "filtering sections by language? Good text-to-speech support? No need for that."
If nothing else, Gemini is theoretically built specifically for generality[0], and it's already guaranteeing out of the box that it can't be used well with voice assistants. Those things aren't a fad, a general-purpose document format kind of needs to support them. And trying to have a voice assistant that intuits the current language is just a really bad system for everyone.
[0]: https://gemini.circumlunar.space/docs/faq.html