I started learning English some time ago, and feel I am a competent speaker. However I've read using Japanese and Chinese makes communication and business much quicker, and theres a less steep learning curve. I know about how dealing with people is easier in those languages, however that doesn't make me want to switch. Essentially, I am asking if I should switch to Japanese. I've succeeded in making a few essays and forum posts in English, and know a lot about the language. I really like English and so far have had fun learning how to speak with it. So should I give up on English and switch to Japanese now that I'm a few months in. (For clarification, English is my first language.
If you read that and thought it sounded strange, well, that just puts your question into perspective.
As has been said: You have the capacity to learn and use multiple languages at once.
If you want to use another language, just start using it, in addition to the languages you currently use.