While Erik is suggesting perhaps the "mainstream" way and there is a wide acceptance of C, let me add a couple of comments:

• you will probably need some good basics of asm knowledge even if you will use mainly high level language (HLL)
  
• there is at least one more HLL maybe worth considering for a novice: Pascal - there is at least one active user of it here :-)
• there is a demo version avilable to try (I don't know the limitations)
  
• BASCOM is a compiler so it might be fast enough, if you prefer BASIC, but I have no experience - any BASCOM user here?
  
• interpreted BASIC might be fast enough, depending on application (this is of course an advertisement of my modification of BASIC52 for RD2 :-) - I don't know what a "localizer" exactly means
  
• as for hardware/chip, this again depends on the requirements, you might be well off with any standard DIP40/PLCC44 12-clocker - there are faster drop-in replacements around
  
• GPS+GPRS - this sounds that you will need at least two UARTS, so consider perhaps a derivative with 2 internal UARTs (not too much of them); otherwise you would need an external UART or bit-banged UART (which most of the users here would not recommend)
  

Jan Waclawek