I know that some couples live in a foreign country but speak the local language at home, although it's not their native language. Why do they do that?

For instance, two English people who live in Spain and speak Spanish among them. Why do they speak a language which is not their native one? Isn't that more difficult?

I know that some couples live in a foreign country but speak the local language at home, although it's not their native language. Why do they do that?
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