This may seem a slightly odd topic for this blog, but I want to suggest that learning profanities in a target language is, at some stage, a necessary part of the road to 'fluency'.
Think of your own native language. Whether you use them or not, and generally I would discourage their use, you no doubt are familiar with a large range of obscene words that are not fit to print. You know what they mean, how to use them, how to interpret them, and even the grammatical and syntactical rules that surround them.
When it comes to learning a new language, it's unlikely that you are going to find this sort of language in your textbook. It may never be appropriate to ask your teacher. But if you are going to approach some kind of native-like competency, you are going to have to learn these somehow even if you never utter a single one of them.
How then to learn? I suggest three methods.
1. Text-based Internet resources
Plenty of language resources exist for profanities on the internet. You can get nice lists of all the vulgar words you should know with a simple google search. This even works for Mongolian. For Latin, there is a suprisingly well-sourced wikipedia page that will give you etymologies and examples from poetry and graffiti. For Gaelic there is a very recent but brief book that covers the topic with some skill.
Assuming you have access to either target-language community life, or at least contemporary television and movies, you can pick up some by simply observing and learning. I suppose if your really felt like targeting certain words, you could find a suitable hollywood movie with a lot of expletives, and then find a dubbed/subtitled local version.
3. A trusted friend
Assuming you have a same-gender close friend native-speaker I think you could approach them and explain that you need to learn these kind of words, and why you need to learn them.
I haven't found a good source for ancient Greek profanity, so if you know one, this is definitely time to leave a comment.