If you set something like "setlocale(LC_ALL, 'de_DE');" in _config.php you'll get values like "0,9" instead of "0.9" for priority in the sitemap.xml and google does not like it (Ungültiger Tag-Wert -> invalid tag-value). Probable one shouldn't use LC_ALL but anyway sitemap should never use a comma instead of a dot. I guess this is a bug or what you suggest me to fill in to _config for having proper translated time & date values?
responding to myself... the ability to read is a nice gift :-)
setting back LC_NUMERIC after LC_ALL to a the default or set each value separate in the first place.