First of all, I'm no expert on SEO.
I'm curious for thoughts on how others have handled SEO on communities that house multiple languages, all on the same environment. Based on some research, it seems like some identifier in the URL (domain.com/fr/something) or alternate domain itself (domain.fr) is ideal.
Lithium Khoros's URL structure doesn't really allow for customizing, so I thought I'd see what others have done.
If you set the node language in community admin this will populate the lang-parameter of the <html> tag of every page in this community section. E.g. for the source of the German Alteryx community at https://community.alteryx.com/?category.id=external-de&profile.language=de you will see that the document language is set to the German language code "de":
<html prefix="og: http://ogp.me/ns#" dir="ltr" lang="de" class="no-js">
This is what search engine crawlers today will rely on primarily to determine the language of a document (with a potential to be overriden if page content is primarily in a different language).
I haven't seen any indication in newer SEO articles that the language code should also be replicated in the URL. A country specific domain is a positive SEO signal though. I haven't seen any community splitting up their language communities to different country level top level domains though.