Many programming languages are available for the PIC18F series of micro-controllers. Many excellent ones are available for free or at low cost. Some options are listed below.
Swordfish Special Edition is a free version limited to several hundred lines of code which is enough for some surprisingly complex programs.
There is a great deal of user-support for Swordfish and an active forum.
The free Amicus Compiler is the full version of the Protron Basic Compiler, but limited to use on the PIC18F25K20 micro-controller. This combination is extremely powerful and can handle complex tasks.
The Amicus system also includes hardware to support many Arduino-compatible shields - the TAP-28 does not support the use of these shields but is a low-cost way of using the Amicus Compiler.
The 18F25K20 is an extremely capable chip, but it's a 3.3V part which may complicate using it.