Phorum 5 i Danish language

I don't understand the answers in this Phorum to problems with language shift.

Problem: After having installed Danish language I get error messages:
Undefined index, e.g. FollowThread in phorum-5.0.11\cache\tpl-default-read-bbef3d4c630b0ae3324dcf247e283b67.php on line 72.

This phorum has answers like insert $PHORUM["DATA"]['LANG_META']=""; - here in danish.php I suppose. But with some content?

Answers also tell to change default header.tbl - but I only get reference to cache\tpl-default-read-bbef3d4c630b0ae3324dcf247e283b67.php. Why?

Maybe not the right place to ask - cf. the Danish language file is here: [phorum.org]