Hello Gunther,

The best place to start is the WPML documentation,

https://wpml.org/documentation/

Two things you need to know.

1. You need to use Strings Translation to translate the theme.

2. For the missing strings you will need to add them to your database of available strings by clicking on the link at the very bottom of the Strings Translation page in WPML, selecting the strings you ant to translate and then adding them.

Once that is done you shouldn’t have any problems.