[BUGFIX] RTE: Words containing umlauts not added to personal dictionary

Problem: The spell checker must analyze the dictionary file and may
have to update the charset of the personal dictionary. In doing so,
it is looking for the wrong file when the dictionary in use is a
regional/variety dictionary.

Change-Id: Ia2b57c0ead7c90a6d40e9db4d396e6c7ded72e5c
Resolves: #29685
Releases: 4.5, 4.6, 4.7, 6.0
Reviewed-on: http://review.typo3.org/15790
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland