Steps to replicate the issue (include links if applicable):

Part 1:

• Define a custom language code in LocalSettings.php which contains -x-, e.g. aa-x-localset.
• Start adding a lexeme.
• Select the custom language code as the spelling variant.
• Try to create the lexeme.

Part 2:

• Disable Javascript.
• Start adding a lexeme.
• Select the custom language code as the spelling variant.
• Create the lexeme.
• Try to edit the lemma or add a form or sense using the custom code.

What happens?:

Part 1:
The language code can be selected in the spelling variant field, but the lexeme can't be created with the error Invalid Item ID "localset" in field "lemma" at "aa-x-localset".

Part 2:
The lexeme can be created, but the lemma can't be edited, and forms and senses can't be added, with a similar error.

What should have happened instead?:

Any custom language codes defined in LocalSettings.php (or Names.php) should be allowed. The QID check should only apply to language codes not already known to MediaWiki.

While Wikimedia is not currently using codes containing -x-, codes such as map-bms and roa-tara might one day be changed to use -x-, and third party users of the extension should not be prevented from defining/adding custom BCP 47-compliant language codes.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

Example line to add to LocalSettings.php
$wgExtraLanguageNames["aa-x-localset"] = "aa-x-localset";