[BUGFIX] l10n_mode for "pages" table and group fields.

This patch respects the l10modes for pages, and mergeIfNotBlank for
type "group" fields.

Change-Id: I18a4caffc5761f91dae4ae0cf175ccd51ffe8c29
Fixes: #38766
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22325
Reviewed-by: Markus Klein
Tested-by: Markus Klein