Multilingual SVG always unexpectedly use the first 2-letter-match, even the specified language exists

List of steps to reproduce (step by step, including full links if applicable):

• Go to https://commons.wikimedia.org/wiki/File:SystemLanguage.svg?lang=ku-latn
• "ku-Latn" is defined after "ku-Arab" in the SVG.
• However, selecting "ku-Latn" return "ku-Arab" as the result of the thumbnail.

• Go to https://commons.wikimedia.org/wiki/File:SystemLanguage.svg?lang=gan
• "gan" is defined after "ga" in the SVG.
• However, selecting "gan" return "ga" as the result of the thumbnail. They are even not identical language code fragments.

What happens?:

• Selecting "ku-Latn" return "ku-Arab" as the result of the thumbnail.
  • Same issue appeared on "ku", which returned "ku-Arab".
  • Same issue appeared on "zh", which returned "ku-Arab".
  • Also for other language codes.
• Selecting "gan" return "ga" as the result of the thumbnail, even when they're not identical language code fragments.
  • Same issue appeared on "azb", which returned "az".
  • Same issue appeared on "ceb", which returned "ce".
  • Also for other language codes.

What should have happened instead?:

• Selecting "ku-Latn" return "ku-Latn" as the result of the thumbnail.
• Selecting "gan" return "gan" as the result of the thumbnail.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.:

• Wiki: https://commons.wikimedia.org
• Version: 1.39.0-wmf.15 ( 6b53b5d )