Considering a modern foreign language GCSE or A-level? Read our FAQ hereWatch
Not only will you be able to communicate more effectively with more people by learning a language, but many transferable skills are connected to language-learning as well. To become proficient in a language, you need the analytical skills associated with the sciences (to learn grammar and get acquainted with the structure of the language) and the cultural and verbal inclination associated with the arts and humanities (to appreciate and analyse the sociocultural environment of the society where the language is spoken). Because of this, foreign languages complement pretty much any subject and are always regarded favourably by universities and employers. I would particularly recommend taking up a language if you want to go into a field like law, economics or international relations, as it could open up some exciting opportunities at international institutions, or if you're doing science subjects and want to look more versatile without having to write lengthy essays.
At A-level, you can no longer get away with barely knowing how to converse, but your level of fluency will similarly depend on your effort. Learning everything on the A-level syllabus will probably get you to a B2 level on the CEFR scale, which will be adequate for most purposes.
A-level is a bit more difficult because you'll have to write and prepare longer pieces (for writing and oral), study literature and potentially even carry out a research project. Once again, though, the workload depends on how you learn languages, how long it takes you to write longer pieces in your target language and how crucial it is for you to consolidate your grammar knowledge frequently. I have heard such wildly varying answers about the workload of modern languages at A-level that there is reason to suppose that it is highly subjective and depends on your personal approach to language acquisition, which becomes more important in your A-level studies as you are given more independence.