To change the default viewer language, open the group options and select "Viewer Customization".
In the "Display & Formatting" tab you can select the language which is applied to all publications in the group.
You can also select a viewer language on publication level. This setting will overwrite the default value set at "Viewer Customization" and will only apply to the selected publication.
You can find the publication settings under the options available on the right in the publication overview (•••).
Supported viewer languages: