In the Regional Settings tab, you can select the language that is applied to all publications in the group.
In the Display & Formatting tab of the publication settings you can also select a reader language which will overwrite the default value set in the Content Presentation Settings menu. The selected value will only apply to the selected publication.
You can find the publication settings under the options available on the right in the publication overview (•••), or by clicking on the publication title in the main overview.
Supported Reader languages: