Russian is a rather widely spoken language. Likewise, you could get plenty of work in many Russian speaking nations as a translator as I'd guess you're a native speaker of English. Although the pay might not be the best =P Most Western nations already have plenty of people who know how to speak Russian (due to migration).
Although, well, once Russia gets over the sanctions, I'm pretty sure that its economy would start booming again, especially if the government turns to immigration to plug in its demographic issues. So Russian would be useful in that occasion.
French is not terribly useful although still a 'prestigious' language. Quite a few African nations have French as a rather popular language in their countries. Africa might be witness to considerable economic development in the coming decades so you could have quite a few opportunities from knowing French if that were to happen. I just know that French has been a very, very difficult language for me to learn, despite my native language being a Latin one.
German is not terribly useful as it's not very widely spoken, although Germany is still an economic giant and I think it's a 'prestigious' language to know too. Perhaps easier for a Brit than French too. Although not an easy language objectively.
Spanish is spoken very widely and South America is still sort of booming economically, and it might continue to do so for a while = opportunities. Besides, you can quite easily build on top of that to learn Portuguese and Italian, so that would be really quite nice.
I'd overall recommend Mandarin, despite it being, I suspect, horribly difficult to learn, at least as long as you don't live in an area where mostly only Mandarin is spoken. Also, Cantonese would be kind of a waste of time if you're going to expend that sort of effort learning a language; Mandarin is the more widely spoken from what I remember.
I would DEFINITELY recommend learning a programming language! It would infinitely broaden your horizons and pretty much every possible employer will like seeing that. So if learning Mandarin or Spanish might not be what you'd necessarily want/like to do, definitely try a programming language.