OP, since you are a translator living in Europe, it would make sense to consolidate your knowledge of European languages. With the EU's translation norms and multilingual institutions, intra-European translation is a solid market, at least for long-term career purposes if you're wary for the time being. Among the languagues you're considering, I think German would be a good asset to have in your portfolio.
Generally speaking, English is a no-brainer for non-native speakers. After that, depends on your goals and what corner of the world you have your sights on. Spanish and Portuguese offer a nice coverage, with both developed and developing countries; Chinese is an obvious pick, political uncertainty notwithstanding. I'm doubtful about Japanese; even though I learned it, I don't think it offers as much in the way of opportunities as the others.
And I'm a bit sceptical about the number of native speakers as an argument for Mandarin. I'm more inclined to mention their prosperity and culture as far as incentives go.