Your mother tongue is the language that you learn from your parents when you are still a baby.
When we were still babies, our parents taught us many words. When we learned to say the word, we made them happy. When I was younger, the only language i was using is ILOCO. But as I grow, I learned different languages like; English, Tagalog, Japanese, Korean, and a little bit of Mandarin Chinese. Even though I use a lot of languages, I never ever forget how to speak in ILOCO. Cause it's the language of the place i came from.
There's a lot of different languages in the world, but we will always have our own mother tongue in our hearths.