People do any number of strange things in their sleep. Have you ever had a weird dream? Heard someone mumble something while they slept? Seen or have been a sleepwalker? The mind can be a mysterious place when it’s supposed to be resting. But is it really possible to talk in a different language when you are asleep? Quite possibly!
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You Subconsciously Know Another Language
Your brain picks up all sorts of tidbits throughout the day. Perhaps you had lunch at a Vietnamese place, or overheard some Spanish at the bank. You may not recognize the words, but you’ve heard them. They may come back out later as you sleep...and talk!
What You're Saying Only Sounds Like Another Language
You are not awake when you are sleep talking, therefore you have no idea what you are saying. Your partner might tell you that you spoke another language, but does that person know that language or just what it sounds like? They might tell you it sounded like you were speaking French when really you were simply speaking gibberish in a French accent.
You Dream in Other Languages
If you have ever immersed yourself in another culture and you know the language well, you might actually have dreams in that language. In those cases, it is quite possible for you to speak another language outside your native tongue while you are sleeping. You are experiencing that language when you dream, so you might talk in it when you are asleep as well.
Sleeping brings you to a whole new world, one very much unlike the one you live in when you are awake. You find you can fly when you are dreaming so it's possible you can speak another language as well. Whether or not you are speaking real words or just gibberish that sounds like Spanish is another matter.