Although many authorities (RL Oxford et al.) distinguish between a second language (one used in the learner's milieu, neighborhood) and a foreign language (a language NOT readily available for everyday, common use; used by speakers out of the learner's accessible neighborhood, nation) effectively learning a foreign language IS learning a second language if you only have one language at the start of the process of learning. Thus we'll examine the benefits of learning a language not absorbed when you were pre-vocal or learned with little conscious, formal study, and probably when you are a young adult or older. In this article we'll use English as the given mother tongue, but the benefits discussed are universal.
One last thought.... In 42 years of teaching languages I have avoided the use of 'foreign' language, even as I taught Korean to English-speakers and English to Thai native speakers and Korean speakers. The whole concept of a 'foreign' language is that it is alien to me, the learner. It is 'foreign', out there, not-mine... and as such, the concept 'foreign' carries some heavy baggage, which freights a learner down much more than need be, so I conscientiously avoid teaching 'foreign' languages to anybody, and encourage them to use self-talk and social dialogue such as "I'm learning my second language," "We speak Thai" or "Korean is MY language."
Because with that by way of introduction, if you CANNOT or will not voice those sentiments to yourself or others, you really need to question WHY you're learning another language at all? If nearly all speakers of your target language are black, and you're not, do you have some racial issues to resolve? Koreans are NOT Caucasians, do you feel uncomfortable calling yourself a Korean speaker? a Thai speaker? an English speaker?
I'm extolling the benefits and joys of second language learning, and THIS is one of the first, identifying and resolving some previously hidden biases and fears, mild xenophobic feelings and general discomforts. We've grown up doing things one way, OUR way, the RIGHT way... and now we want to learn a language that does NOT use our mother tongue to move ideas from one human to another... and we start realizing we're moving OUT of our comfort zone.
That movement is a GOOD thing, inasmuch as humans who are too comfortable are often in a rut! Learning your 'foreign' language will move you out of your rut, I promise you that.
Another benefit comes with your struggle to grasp and internalize new figures of speech, nuances and ambiguities. The new language, slowly at first and then with more potent effect, helps you sort through your values, your habits and your culture to evaluate or re-evaluate what is worthy and what might be better left undone.
A corollary to this is that in learning your erstwhile foreign language, you learn English more deeply, more meaningfully and this enriches your life immensely. Some languages use prefixes and suffixes as English does, and others use infixes extensively. Seeing clearly what English is NOT helps clarify your knowledge of what English is! Learning a different grammar, word-order, syntax and vocabulary enriches the learner's own base language.
Finally, learning your second language and using it in the real world, real-time, lays a solid foundation for learning your THIRD language, even if its a language used halfway around the world! You now have TWO languages, two sets of sounds and meanings to use as hooks for learning your third set of sounds and meanings! And now you will be learning the strengths, hopes, fears and jokes of a third group of humans eager to share their learnings with you, the eager student of THEIR mother tongue!
Bon appetit!