There are a lot of language learning apps out there (my favorites are Drops and Memrise), but one immersive way of forcing yourself to learn a language, short of traveling to another country, is to go to your iPhone/iPad Settings->General->Language and Region and then dragging the language you want to learn to the top (in Android, it's similar but you have to scroll down and hit Apply)

Soon you will learn at least how to recognize Cancel and OK in another language (Pro tip: learn how to recognize Language and Region in this new language or you'll have a tough time switching your device back to English)

And if you choose a fairly popular language (in terms of important market demographics), you can also practice with well-localized apps.

In fact, I started this device language switching to test localization in my own app development. And you can see non-localized or partially localized apps stick out like a sore thumb.