To avoid practical complications (e.g., the RecentChanges patrollers are confused because |thumb appears in dozens of languages and they can't know which ones will work and which ones are nonsense/typos), I wonder whether the namespaces/wikitext codes could be autotransformed upon save, so that you can type [[Bild:Image.jpg]] into enwiki, but it would still save as [[File:Image.jpg]].

Hmm, during our discussion about this, I was thinking of it more as "teaching/reminding/encouraging language communities about how to utilize the various existing localization features".

• E.g. At a small-level: Zh wikis don't have "File" namespace localized whereas the Ja wikis do. The simplest solution is to point a community's tech-ambassadors/translators towards the docs at https://www.mediawiki.org/wiki/Localisation#Localising_namespaces_and_special_page_aliases (which say "write a phab task") in case they want that.
• E.g. ditto for ultra-common parameters like |thumb and |left etc, which some communities have localized and some don't. I don't know where those get translated.
• E.g. At a large-level, this could involve improving the existing docs (?!) that new/small communities are pointed towards. I don't know what he currently direct them to, beyond https://translatewiki.net/wiki/Translating:MediaWiki#Recommended_process_for_translating_MediaWiki
• E.g. At a "needs-developer" level, we could encourage work on T109235: Re-enable Special:AdvancedTranslate on translatewiki.net which seems to be how that kind of thing used to be done, until something broke.

However, I do not think MediaWiki should accept all translations at all wikis. It would be very confusing to have 300+ ways of writing the same syntax, all potentially in use at a single wiki.
But I do like Whatamidoing's suggestion about autotransform, though am not sure how difficult that would be to implement vs benefit. Perhaps this task could be re-purposed to that, or similar?

No opinion on whether this is a good idea. But I must note that it is impossible, because some of the translations conflict. I checked this using some silly regexes, so this may be incomplete, but at least:

• The namespace name 'Stampa' means:
  • 'Template' in languages 'aln', 'sq'
  • 'File' in language 'mt'
• The namespace name 'Datoteka' means:
  • 'File' in languages 'bs', 'hr', 'sh', 'sr-el'
  • 'Media' in language 'sl'

So you'd have to change these translations first (probably also convince the relevant communities that it's a good idea), then fix the existing content.

@Quiddity:

• E.g. At a small-level: Zh wikis don't have "File" namespace localized whereas the Ja wikis do.

Nearly all namespaces of zh.* projects are using English namespace names, except Module: and Module_talk: (which are unfairly localized as mainland "模块:" and "模块讨论:", T165593), per T199577#4424881, translations are defined and shown as aliases by InitialiseSettings.php separately.
The reason why Japanese Wikipedia don't translate their namespaces, IMHO, is interesting, as other Japanese projects are really using localized namespaces.