For the organ in your mouth:
* Lingua (formal, scientific)
* Glossa (formal, scientific)
* Palate (can refer to the roof of the mouth, but sometimes used to mean "tongue")
* Mouth (broadly, referring to the whole organ)
For the language someone speaks:
* Dialect
* Lingo
* Vernacular
* Idiom
For the "tongue" of a shoe:
* Lapping
* Tab
* Strap
For the "tongue" of a bell:
* Clapper
* Hammer
For the "tongue" of a buckle:
* Pin
* Tongue (this is actually the most common term for this)
To get a more accurate synonym, please provide the context of how you're using the word "tongue".