How to handle possibly subtly flirtatious emails from students?
You should keep your replies short, polite and professional. Do not respond to what you see as flirtatious. Try also to interpret what you see as flirtatious in a different light, as you say, it could be due to language issues. It would be bad if you started to respond in a manner that assumed something that was not intended.

In the end it is very important to not fuel any behaviour that you think is "suspect" but at the same time you cannot avoid responding altogether. Acknowledge praise briefly but do not return praise since that could appear as favouring a student before others. Turn the focus of a response quickly from any polite exchange to focus on the course material. You may also take opportunities to point out that information given is also given to others to at least subtly impress the fact that there is no "special treatment".

In short, act as if nothing special is going on, be brief to the point and professional in your response. Do not try to be anything other than yourself or treat any student differently from others. Keep records of your mail or other exchanges.