There is only one file type that uses the .TMLANGUAGE extension:
TextMate Language Grammar File
Language grammar file used by TextMate, a Mac OS X source code editor; contains a specification for the syntax of a programming language; used for adding syntax highlighting support for additional languages.
TMLANGUAGE files are saved using XML formatting but may be encoded with a binary representation of the XML data.
NOTE: TextMate language grammar files are often saved as binary Property List (.PLIST) files. For example, some of the .TMBUNDLE files included with the TextMate installation include a PLIST file in their /Syntaxes/ directory.
Updated: October 24, 2010