[BUGFIX] Cache file could not be written on concurrent actions

The hook to clear the language (l10n) caches is currently called
every time any element in the backend is modified. If concurrent
actions take place, flushing and regenerating this cache results
in an accordant exception like

The cache file ".../t3lib_l10n/3e2cbbda0301cf592e5831ef26c56b7b"
could not be written.

The solution is to only execute this hook if all or the temp.
caches shall be cleared

Change-Id: Iaf68f50e2f9bfcc3ffce64a0f5267673afd332ad
Fixes: #46205
Releases: 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/19904
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer