Wikibase uses currently the URL parameter `setlang` in:
* https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FWikibase.git/3384bd90c81a44fba739aa4961237402c4213214/view%2Fresources%2Fjquery%2Fwikibase%2Fjquery.wikibase.entitytermsforlanguageview.js#L63
* https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FWikibase.git/3384bd90c81a44fba739aa4961237402c4213214/view%2Fsrc%2FEntityTermsView.php#L223
`setlang` is problematic because
* setlang is suggested to deprecate by T111990.
* setlang does not work for anonymous user.
* signed-in user normally do not want to change their complete user interface language. They just want to see the content in an other language
I suggest the following alternative changes:
* Replace `setlang` by `uselang`. This changes the user interface language only for a single click.
* Replace the link to an URL by a call of the language change API of ULS.
* Introduce a new content language which defaults to the user interface language and can set via user options, URL parameter.