Implemented the restriction in request #27894.

A simple workaround can be to reduce the list of languages. This can easily be implemented in the "TuleapIntegration" extension.

Example Code to try out in LocalSettings.Tuleap.php

$GLOBALS['wgHooks']['GetPreferences'][] = function( $user, array &$preferences ) {
	$preferences['language']['options'] = array_filter(
		$preferences['language']['options'],
		function( $langCode ) {
			return in_array( $langCode, ['en', 'fr', 'pt'] );
		}
);
	return true;
};

I have tested this on our internal testsystem. The user preference is properly transferred from Tuleap and persisted into the Wikis DB (A re-login/session-reset may be requried though).

But: Currently this change does not take any effect, as LocalSettings.Tuleap.php contains

$GLOBALS['wgHiddenPrefs'][] = 'language';

In such cases, MediaWiki will fall back to the default value, which is the same as the wikis content language. We must not modify the content language based on a user specific preference, so we have to remove that setting.

If you need to hide the "Language" option from "Special:Preferences" we may try to use CSS in the skin to just hide it, or come up with another programmatic solution.

Which do you prefer?

Done in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/TuleapIntegration/+/809931