I am not sure really sure that I understand @Tordanik’s reasoning here. For the end user the URL is
unimportant. Virtually nobody will look at it or even enter it manually somewhere. URLs without the name slug also work (e.g. https://community.openstreetmap.org/c/communities/61), so APIs are no issue. For these reasons, I am looking at the problem (almost) exclusively from an SEO perspective.

Good question. I guess the simplest approach would be to just us all of them:
https://community.openstreetmap.org/c/communities/belgie-belqique-belgien-belgium/61

If you consider it being too long, I think the language that has the most speakers in the country should be chosen.

You are comparing apples and oranges. The Wikipedia subdomains are used for language versions, not local communities. They are unimportant as search engines don’t need the URL to tell them the language of a website, they can see it from source code or (if missing there) from the text.

Without information from Google Search Console or a similar service, it is not easy to answer the question, what search terms people use to find this forum. In general, this forum is not very discoverable yet, as can be seen with a few tests:

  1. When I search “OpenStreetMap Germany” in German Google, the old forum appears on page 2. The new forum is nowhere to be found.
  2. When I search “OpenStreetMap Deutschland”, neither of them is found in the first ten pages.
  3. “OpenStreetMap forum” has old forum in position 1, new forum nowhere
  4. “OpenStreetMap community” has new forum in position 2

Thoughts about that (and some statistics):

  1. As the new forum is using a new subdomain, it has not generated much reputation yet and not many backlinks. This can be one reason for bad ranking that will improve over time.
  2. In Germany, more people search for “OpenStreetMap Deutschland” than “OpenStreetMap Germany” (source). For that reason, the old forum was not very discoverable, this should be done better here.
  3. Worldwide, more people search for “OpenStreetMap Forum” than “OpenStreetMap Community” (source). The name and subdomain of the new forum are therefore badly chosen from an SEO perspective.