[BUGFIX] RTE: Incorrect text in language combo of abbreviation dialogue

The language combo of the RTE abbreviation dialogue is not correctly
initialized when no language is selected. ExtJs bug?..
Solution: Change the way the language store is loaded.

Releases: master, 6.2
Resolves: #63038
Change-Id: Icba6441f2a708254eb6f7f7f1a592ab42da59e55
Reviewed-on: http://review.typo3.org/34278
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>