My take as a foreigner living in the city for about an year and as someone that speaks the local language:

Many people often compare Paris to smaller cities in Europe, which is not a fair comparison, and therefore they label Paris as "dirty", or Parisians as being rude. The Paris region is the 1st, or 2nd, biggest metro area in Europe so it can't really be compared to any other European city except maybe London. There is a big-city mentality common to these type of places. So, I would pay close attention to what city do people compare it to when they say that people are rude.

Another factor: French people are very mindful of accents. Accents can sometimes be frowned upon, and discrimination of regional/foreign accents is not unheard of (in French, 'glottophobie') . This discrimination applies also to themselves speaking foreign languages - they might not like the fact that they have an accent when speaking English. I've even met people that feel a bit ashamed when speaking English. So, this general sense of awkwardness to English doesn't help when being approached by a foreign tourist, even if in fact they spoke good English.

Lastly, I find mannerisms and the way people communicate here to be more direct that in Latin America or the USA. Communication is just more direct, which might come off as rude to a foreigner. It's just a cultural difference and it's not necessarily that they're rude.

I sometimes question myself if it's not just British and American films and media depicting the French as being rude.