[TASK] Properly select Chinese (Simplified) in DatePicker.js

The language identifier key for Chinese in TYPO3s backend is ‘ch’.
The valid identifier should be ‘zh-cn’, but we cannot change the entire translation server
in a meaningful way.

Releases: master
Resolves: #79179
Change-Id: I5fee14f2eceacc5676588bdb8cac2aeceb88fff6
Reviewed-on: https://review.typo3.org/51161


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>