It depends on your field and on the kind of companies you want to work with. French companies, foreign ones? For example, if I want to work with French companies in my field, German is almost useless for me. Yet Germany is our biggest trade partner and I could also decide to go to Germany and find a local job. It all depends on what your goals are, so learning a language only because someone will have told you that said country is booming isn’t very wise in my opinion.
Russian is spoken in more countries and is closer to us than Korean, but I’m sure there are some good opportunities for Korean speakers too. Would you rather live / work in Russia / Central Asia or in South Korea? Both languages seem to be hard for different reasons, but if your Japanese is good Korean might be easier for you.
Learn the one you like the most, and you’ll find job opportunities accordingly.