[BUGFIX] Numeric translation keys aren't translated right in XML files

When working with the "old" XML based translation files,
numeric keys aren't translated properly.
The reason is, that LocallangXmlParser uses array_merge to combine the
translation arrays of the default language and the current language.
array_merge renumbers numeric keys. Therefore, numeric key in
translation files result in wrong translations in Frontend.

Fixes: #44626
Releases: 6.0, 6.1
Change-Id: I8fdff385f681428b4cb5a3e994828a4a29305c98
Reviewed-on: https://review.typo3.org/17615
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf