English Only - Not sure if this kind of 'condition/validation' would work, but try something like:
string-length(translate(., "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ.,", "")) > 0
Make sure you include all 'legal' characters in the first string. For e.g. the above as validation would fail if I put numbers in the field.