In school, we only had German, English, French and Spanish classes. Whenever we complained about French being so complicated, our teacher said there are much worse languages in this world 
A friend of mine took an extra class to learn Chinese and I realised how hard learning languages can be if you need to learn new letters 
Could there be something even more difficult?
What do you think would be the hardest language to learn?
French is a pain because of all the different tenses along with pronouns and everything. I think that Chinese or Japanese would be super hard though, even Korean would be way to much for me.
speaking from experience, ancient greek is A PAIN to learn because of all the grammatical cases and tenses
however, i’d say English
since English grammar can be quite messy: stealing grammar rules from many different languages and not having all of them match up logically, because of this there’s A LOT of exceptions , plus the pronunciation of each letter can change a ton
also, since it’s so widely spoken in online spaces, it changes so fast, it can be hard for a non native to catch up
I’d say maybe Japanese with all that Kanji and different alphabets they use 
Korean looks difficult too but learning the alphabet was pretty easy for me, the pronunciation is just -
All the languages.
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