Hi Christian,
The keywords you mention for the "switch" attribute are just prefixes, not a list of allowed values. It would not be easy (if at all possible) to define constraints in the corresponding XML Schema for the module.
For the "case" attribute, the situation is more complex because the attribute values must also be constructed taking into account the associated "switch" attribute, keeping in mind that the possible values for "gender" are an open list.
XLIFF validation cannot be done with just a validating XML parser. You need code that checks for compliance with the constraints described in the specification document.
While I agree with you that it would be nice to have the keywords in a machine-readable format, we simply have to accept that validating attributes related to gender, plural, and selection requires code that applies some logic to verify compliance with the constraints defined in the corresponding module.
Regards,
Rodolfo