Should you learn the most common language, even if it isn't the official one?

Most countries have either an official language or a de facto language. But what about on a smaller scale? If another language is more prevalent in your state or area, do you have a responsibility to learn it, even if it's a foreign language?
Should you learn the most common language, even if it isnt the official one?
Should you learn the most common language, even if it isn't the official one?
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