Subforums seem like a step backwards, to me. For one, it doesn't effectively solve the problem of languages for which there may only be novice users, without any experts to provide guidance. But even beyond that, dividing the forums like this seems antithetical to building a unitary Swift community. Non-English speakers remain discouraged from participating in Swift Evolution discussions or other primarily-English threads. Building language detection into forums in a per-post manner would still potentially allow users to filter posts by language, or hide those that aren't in their native language if they're not interested in reading translations.
There are plenty of solutions available currently (self-translating, copy-pasting to machine translation services, etc.) that empower those who want to put in the effort to collaborate across languages, but they are high-friction and potentially increase noise in the forums. I think it's indicative that non-English threads today generally don't have participants entering to ask the poster to translate.
This is precisely the issue that this thread intends to address. Obviously, in the happy path where we have knowledgable community members who are able to provide useful answers in the language in question, there's no issue. But total silence is, IMO, a very poor signal. It could be an indication that nobody speaks the language in question, but it could mean countless different things as well. If it is a goal of the Swift project to support users who speak many different languages (which, as @Max_Desiatov points out, it appears to be), the forums should strive to support those community members as well.
There's sort of a chicken-and-egg problem here—would the forums see more more non-English usage if localization support were better, or does the relative dearth of non-English posts indicate that it would be wasted effort/investment? I don't know how to answer this question, but if effort is being invested to expand the usefulness of the Swift compiler to non-English speakers, we should expect to see an increase in non-English speakers coming to the forums.
Of course, there's always going to be some inherent friction in cross-language communication, and I don't think that there's any realistic risk that decreasing that friction would cause English to cease being the lingua franca of the Swift forums.
A lot of this may require development support to e.g. build a custom plugin, or expand the capabilities of Discourse, so maybe it's too pie-in-the sky for now...