sys_language.static_lang_isocode has been removed

[bernd-reindl]

sys_language.static_lang_isocode has been removed in TYPO3 10 (https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/10.0/Breaking-88583-DatabaseFieldSys_languagestatic_lang_isocodeRemoved.html)

So \DirectMailTeam\DirectMail\SelectCategories leads in an errormessage when a content element is edited.