It depends what you understand as career.
If you have work as employee in your country, and you want it to stay so, there's usually no need for you to know any language except your native and English. An exception may be, when your company's main client is foreign company, and you should know the language of that company. For example, many companies from west Poland are cooperating mostly with German companies, and knowing German is a great advantage, or even requirement, for working as senior developers (while juniors don't have contact with customer, they are not required to speak German).
However, if you want be a freelancer, you need to be fluent in the language of your customer. You can of course target English-speaking countries only, but knowing for example German or France gives you more opportunities, and potential better payed contracts.
If you want to work abroad, it depends. Of course, going to USA or UK, the English is the correct choice. In small countries (in Europe) there are many English-speaking teams, so as a foreign contractor, you can hope for many English-speaking jobs for example in Denmark or Netherlands.
But in big countries such as Germany, English-speaking jobs are an exception, and without knowing German good you have little chances of finding something. To get best payed contracts, you need to speak German fluent, but you can then get really good payed jobs.