I got this response:
您好：108年5月21日的電子郵件已收到。
按就業服務法第5條第1項規定：「為保障國民就業機會平等，雇主對求職人或所僱用員工，不得以種族、階級、語言、思想、宗教、黨派、籍貫、出生地、性別、性傾向、年齡、婚姻、容貌、五官、身心障礙、星座、血型或以往工會會員身分為由，予以歧視；其他法律有明文規定者，從其規定。」。
又「就業歧視」係指當雇主以求職人或所僱用員工「與執行該項特定工作無關之特質」決定受僱與否或其勞動條件，且雇主在該項特質上之要求有不公平且不合理之情事，可認定為雇主對求職人或所僱用員工之歧視。
所詢語文補習班聘僱教師限美國國籍，是否涉就業服務法第5條第1項規定，須依個案事實認定，建議敘明詳情洽請事業單位所在地勞工行政主管機關(直轄市、縣市政府勞工或社會局、處)協處，較為迅捷。
The question in my mind is that “nationality” is not listed as a protected characteristic in the law. If it was, this would be easy. The Labor people’s usual MO with these kind of questions seems to be “eh, um, maybe, uh, report the incident to the proper authorities”. They have followed this pattern here, and haven’t provided any clarity as to whether this is an illegal practice. As for the second paragraph, it seems to me there may be reasons which they would deem as being “related to the work” and not “unfair”. My conclusion here is that at least with regard to the law I don’t see a clear reason for us to prohibit this kind of wording. I welcome any opinions.