Why turbo c is program in c lang?

Your question makes no sense.



Actually it makes perfect sense and is a very good question, most modern compilers are written in the language they compile. It makes them easier to update, test, and maintain. C compilers are written in C, Pascal compilers are written in Pascal, Ada compilers are written in Ada, Fortran compilers are written in Fortran, etc. If the old compiler can successfully compile the new compiler and the new compiler can successfully itself, it makes a very effective and easy to run test case that the new compiler works correctly.

Compilers don't have to be written in the language they compile, but it has become almost general practice to do so, as explained above.