I'd like to start a discussion about the merits and challenges of
enabling the Translate and UniversalLanguageSelector extensions on
Commons.

Thanks Guillaume for starting this discussion.

The Translate extension (
https://www.mediawiki.org/wiki/Extension:Translate ) is super useful
to translate content; it's already used for translations on Meta,
mediawiki.org and other non-Wikimedia wikis. In my experience, this
extension usually increases the amount of translations and
multilingual content on a given wiki, which Commons would benefit
from.

For my little experience with the way the extension works on Meta, I see how it works to translate single pages, or messages assembled in a single page − this could indeed be very useful for all our multilingual pages (policy, help, etc.)

Could the extension can, or be tweaked to, help with our other multilingual content? I can think of:
* templates (whether using Autotranslate/Fallback, LangSwitch, or special constructs like {{Technique}})
* file descriptions ?
* POTD descriptions ?
* SiteNotice announcements ?

The UniversalLanguageSelector extension (
https://www.mediawiki.org/wiki/Extension:UniversalLanguageSelector )
allows users to easily switch the interface language; it also embeds
an interface to enter text in non-Latin languages even if your
keyboard doesn't allow it.

Seems very useful indeed.

Just asking: would it play nicely with our AnonymousI18N script?
<https://commons.wikimedia.org/wiki/MediaWiki:AnonymousI18N.js>

Incidently, Rillke has openend a section on [[COM:VPP]] on this.
<https://commons.wikimedia.org/wiki/Commons:VPP#Elaborate_whether_mw:Extension:UniversalLanguageSelector_is_suitable_for_Commons>

There would probably be some adjustments to make to transition from
the current way we handle multilingual content and interface, but I
don't perceive them as insurmountable.

Yes. Indeed, I wondered above how the Extension would fit with our black magic, but we can certainly think of revamping our magic to benefit from the extensions.