Where I'm from originally, I was part of a rather large French Canadian community. French is our mother tongue but we all learned how to speak English, fluently because the rest of the community, which out numbered ours spoke English as their first language. Could we have survived and found work had we not been willing to learn English? Probably, but why make things harder on yourself than you need to?
Besides, knowing two languages, both of which happen to be Canada's official languages opens more doors for me had I only spoke French.