Essentially, any question that is not job-related cannot be asked, such as asking (or commenting) about a candidate's age, sex, race, national origin, religion, marital or familial status, disabilities, or health status.
However, if the ability to speak Spanish was determined to be an important, job-related task in the job analysis (for example, the job incumbent will need to be able to address customer concerns in both English and Spanish), then yes, it is fine to ask whether the applicant is fluent in Spanish. It also would be appropriate to ask an applicant to describe situations in which he or she has used Spanish in a business setting and for what purposes.
It should be noted that, in general, the interview should not be used to assess the applicant’s level of fluency by requiring speaking in Spanish unless the interview was constructed for that purpose only and the method of scoring has been carefully constructed.
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