You must learn assembly language first because the 8052 is not a very fast mcu for real-time tasks. Later on, you can learn C for math routines(numerical methods), complex algorithms and data structures.(e.g. records, linked lists, sorting, searching, etc.) and other things can be done normally.

Learning other languages might just waste time since very few use them(non-standard so references are hard to find) and practically no job offer.