English is the lingua franca, Japanese is for all the weeb shit we like.
Then Russian is very useful for piracy purposes and other non-abiding stuff, if you're into that.
Arabic and Chinese are also good assets to your toolbelt - there is a whole Internet parallel universe once you understand these languages, unavailable to foreigners.
Spanish (and/or Portuguese) would be next level, especially Spanish for the piracy. Although it's mostly stuff reposted from China, USA and Russia, their community is important due sheer numbers.
Then comes languages with online presence, but not so much to offer - Korean*, Polish, Tagalo, Malay, Thai, Indonesian, Hindi.
There are also shrinking languages that still hold some minor relevance due past cultural achievements, like German, Italian, French.
I guess that's about it. Did I forget anybody?
*Korean is arguably "as useful as Japanese", but that only applies to teenager girls who like k-pop and doramas. Both South Korea and China are eager to steal Japan's throne in the entertainment industry, so keep an eye open for these languages.