[BUGFIX] Reset array keys after filtering available languages

To prevent PHP from encoding the numeric keys
of the available languages as string, which would
lead to JavaScript errors, `array_values` is now
applied to the filtered list of languages.

This resets the keys and therefore ensures a
continuous numeric sequence starting from 0.

Reference: https://www.php.net/manual/en/function.json-encode.php#refsect1-function.json-encode-notes

Resolves: #102911
Releases: main, 12.4
Change-Id: Ieca55dd815d1389f44823b252df8da75196d0fdc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82734


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>