Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

I mean needing it for work or because of moving.

Languages like English (as a second language) are usually learned because they enable you to connect more, its however no necessary thing where you live. English opens up about half the internet for you, Spanish another 30% (not actual numbers) and they aren't hard to learn.

Russian however is not spoken outside of Russia and many Russians (especially Internet Russians do speak English)

TLDR

English is a Low investment high return language

Russian is very high investment low return.

There is no "hardest" language.

It all depends on a lot of stuff but one very important part is what language you already speak.

Learning Germanic languages like Icelandic is easier if you speak a Germanic language like English and especially one of the Nordic languages.

Learning languages that are not related to your own language is usually harder.

I said two of the hardest, not the hardest.