Change your first language

"What was your first language?" was a question in a recent questionaire that I read.

It led me to think: does one change one's first language?  Is that possible?

I subsequently posted my question on Facebook.

What my friends answered was a revelation to me.  Here is a summary of what they said:

• "First language" implies "native tongue" and fluency in that language.
• Her husband is fluent in four languages.  His first language is changable, depending on who he is talking to.
• She has two first languages, and has a hard time deciding which is really her "first".
• Another friend claims English as his first language.  He has since learned two other languages, and is quite fluent in all three.  He claims that his heart language is not his first language.
• Yet another friend picks up languages quite quickly.  She said she learned Indonesian because she thought she would live there, and Danish because her husband prefers to speak to her in that language.

Is this your experience with languages?  What does "first language" mean to you?  Do you have experiences to add to my list?

Please share.  Thank you.