Open swagger via
https://<yourserver:port>/app/swagger-ui/index.html#/user-session-rest-controller/setLanguageUsingPUT
Enter this code:
{
"key": "en_US",
"caption": "null"
}
{
"key": "de_DE",
"caption": "null"
}
{
"key": "de_CH",
"caption": "null"
}