Abroad, French people and especially Parisians are infamous for their perceived aloofness, which often is mistaken for an infuriating arrogance.
But the French 'I don't care' attitude is not necessarily a symptom of a superiority complex (at least not always) and can be just a byproduct of the generally slightly lower level of enthusiasm in France compared to some Anglophone countries (French people rarely exclaim things like “oh my god, that was so awesome”).
However, the French language has a long list of different ways to express indifference – ranging from the family-friendly to some slightly more robust ones.
Here's a few examples.
1. Je m’en fiche
This is perhaps the expression you will hear used the most. Along with ça m'est égal, je m'en fiche pretty much is the queen of the French 'I don't cares'. It's not vulgar like its sister expression je m'en fous, just a little colloquial.
To foreigners, it can be a little confusing that je m'en fiche also can mean 'I'm easy'. If you ask your new French flame whether they prefer a dinner or a movie and they respond je m'en fiche, it doesn't necessarily mean they don't care about the date – they're just saying 'up to you'.
2. Ca m'est égal
3. Je m’en fous
Your boss might, however, frown upon you saying je m'en fous. This one, common albeit (very) colloquial, might best be translated to 'I don't give a shit' in English (or something a tad ruder). It's perhaps wise to refrain for using it in front of French in-laws, old ladies, or others who potentially could take offence, but it's pretty safe to use among friends.
4. Je m’en moque
Another politer version. Je m'en moque means 'I couldn't care less'. Moquer means 'to mock', so this expression is playing quite well into the stereotype of arrogant, huffing Parisians with their nose in the air. Te moquer de quelque chose means 'it's not of interest to you' or 'it doesn't affect you'.
5. Je m’en tape
This is like je m'en moque, just a bit more colloquial. Taper means 'to hit', but in this sense its more a referral to se taper, 'to bang' – so it's sort of like saying 'I don't give a toss'.
6. Je m'en cogne
Similarly, je m'en cogne translates directly to 'I don't give a thump' – cogner means to 'bump', 'knock' or 'hit' something – and also is a way of saying 'I could not care less'. You can also say, j'en n'ai rien à cogner, which means the same.
7. Comme tu veux
Another respectful way to say you don't mind is comme tu veux – 'as you want'. This is perhaps the sneakiest French way of disguising indifference as politeness (if that's what it is), because you push the responsibility of choosing onto the other person. Say you're selecting a movie and your partner asks which one you'd rather watch. If you respond comme tu veux you're both saying 'up to you' and 'I don't mind' – which seems very nice of you.
However, the trap here is that it's difficult to know whether a person actually doesn't care when they say comme tu veux. Take the example above just with the roles reversed, so it's your partner who says comme tu veux and lets you pick the film; it might be that they actually don't care, but maybe they're secretly thinking “I'll dump her if she chooses Bridget Jones' Diary again”.
8. Je n'en ai rien à foutre
This is a much clearer and much more vulgar way to say you don't care. It can be loosely translated into 'I have nothing to add', but a good English equivalent could be 'I don't give a tiny rat's ass'. This expression is often used to disregard someone's counter-arguments or excuses. Say your friend is late for your group run – like he usually is – and has sent everyone a long text explaining his delayed train in excruciating detail. If you burst out, Je n'en ai rien à foutre, il a intérêt à être à l'heure ! you're saying 'I don't care about all that, he needs to be here on time!'.
9. Je n'en ai rien à cirer
This is a politer version of ne rien avoir à foutre. It means the same – 'I couldn't care less' – but it's not offensive. Cirer is not a gros mot (swear word), it just means 'to polish' or 'wax' (like what you do with leather shoes or fine wooden floors). It allegedly dates all the way back to the 15th century, originating at sea. When French seamen finished polishing the ship and their superior asked them to get back to work, they would say on n'a rien a cirer – we have nothing to wax.
10. Je m’en bats les couilles
Oh-là-là, we've gotten to perhaps the rudest, least parent-friendly one on this list. Je m'en bat les couilles involves both slapping and testicles and should not be used unless you're among friends. Perhaps its closest English equivalent is 'I couldn't give a flying fuck'.
This one is to be saved for the times when you really don't care. In fact, you care so little that it's a little infuriating that someone would even THINK you care.