[BUGFIX] Avoid undefined array key warning in LanguageService

Guard invalid array key access in LanguageServices, also it
relates to an invalid xliff translation identifier. However,
the PHP warning should be guarded properly.

Resolves: #99190
Resolves: #98713
Releases: 11.5
Change-Id: Ied1894b4863804cfda4840379c0472cf3807a4d2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76838


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>