MergeIfNotBlank should work for pages
The l10n_mode mergeIfNotBlank should have an effect in the frontend for the pages / pages_language_overlay; currently the fields are simply merged for the pages table
is_array($row) ? array_merge($pageInput, $row) : $pageInput;
Additionally, "blank" is not recognized as such for group fields currently, as they have the "0" value. Example is the "shortcut" field for pages.