Change 191250 had a related patch set uploaded (by Spage):
Make Special:ApiHelp query string parameters work

https://gerrit.wikimedia.org/r/191250

Patch-For-Review

Maybe I'm overthinking this :) If a French user

1. creates an account on mediawiki.org
2. sets Preferences > Language to fr - français
3. visits a page that transcludes Special:ApiHelp , such as https://www.mediawiki.org/wiki/Extension:TextExtracts

then the page shows the transcluded API help in French, regardless of the language of the parent page. Try it, follow these steps and instead visit https://www.mediawiki.org/wiki/Extension:TextExtracts/es , the Spanish page also transcludes the French API help. Known French users get API help in French \o/.

So the only use case addressed by the non-trivial work to change SpecialApiHelp.php this bug and customize templates is this:

1. an anonymous user visits mw.org
2. browses to https://www.mediawiki.org/wiki/Extension:TextExtracts ,
3. notices the French translation and clicks https://www.mediawiki.org/wiki/Extension:TextExtracts/fr

but the English API help gets transcluded because we can't tell Special:ApiHelp to do otherwise.

I asked the mw.org i18n community here how common the second scenario is.

CCing @Nemo_bis as he might have a well founded opinion on this.

In T89768#1085383, @Spage wrote:

I asked the mw.org i18n community here how common the second scenario is.

Well, do you expect people not involved in the MediaWiki project to create an account just to see the help pages in their chosen languages? Or reply to your question about how common is this scenario? Even on the Project:Support desk there are a good number of people that prefer to post without creating an account, without the benefit of having notifications of replied posts or having problems with captchas

Ugh, thought I had saved my reply yesterday.

I asked the mw.org i18n community here how common the second scenario is.

Logged out visits to manual pages are certainly the majority, like everywhere else. Only a little minority of "devs" and API users is regularly active and logged in on mediawiki.org. T58464 is indeed a very important bug; if that doesn't get solved in few months, I suggest that you/we adopt Commons' and Wikidata's solution for unregistered users.

Either way, setting language by subpage/transclusion parameters is a valid request.

In T89768#1090162, @Ciencia_Al_Poder wrote:

(thanks for your work on mw.org!)

Well, do you expect people not involved in the MediaWiki project to create an account just to see the help pages in their chosen languages?

This is for Special:ApiHelp, it's fairly specialized and is not yet in wide use, and the parameter names will still be in English. But I agree it would be nice to provide a consistent experience if a user visits a translated mw.org page with API help in it. I also filed T93314: language selector for api.php help to let the user choose when viewing api.php help output.

In T89768#1090837, @Nemo_bis wrote:

T58464 is indeed a very important bug; if that doesn't get solved in few months, I suggest that you/we adopt Commons' and Wikidata's solution for unregistered users.

I wasn't aware of AnonymousI18N.js. Is there a phab task to adopt it on mw.org? I

I wasn't aware of AnonymousI18N.js. Is there a phab task to adopt it on mw.org?

No, a discussion in Project:Current_issues is the way to go.