Does anyone have best practices for when (and when not to) use bi-lingual signage in retail locations?

Bilingual makes sense as long as you know that it serves best your audience. If it's is all about spicing thing up, you should consider whether or not it fits your brand (e.g. PM communicates all commercials in English here in NL). 

Yes, in my past work experience I managed the marketing for retail stores and shopping centers in a bilingual region. We saw that it is bilingual signage worked better than just using english. It is important to give the same dignity to both languages (same font, visibility ecc.). If not people could feel offended or not included properly.