You have a few options for getting C built to run on an X68.
- XC: Sharps official tools. Not recommended for a lot of reasons.
- GCC 1.3: This is the recommended toolchain. It seems to be the most overall compatible and working C toolchain and has regparm support which is pretty nice.
- GCC 2.6.3: I finally got this to produce a working DooM binary but it has an unidentified problem in the raycasting part that isn't present when built with 1.3. Because of this I can't recommend you use it.
- Lydux GCC 4.6.2 cross tools: This can be a workable solution if you are only doing simple C programs or carefully writing assembly. The newlib port is very incomplete and it seems to have problems related to relocations so compiling very complex things(DooM...) isn't going to work.
There are also considerations around what assembler to use if you aren't using Lydux tools. All of the on machine tools use one version or another of Sharps AS or HAS. These assemblers are similar to GAS but they're a bit wonky in places. Output from the disassembler DIS should, but won't always, rebuild using some version of HAS. DIS doesn't seem to produce correct output all the time but there are myriad options and maybe it's workable for whatever if you poke at it hard enough. The nice thing about DIS is that it marks IOCSCALL, DOSCALL and other things in the disassembly which can make reading the output a bit easier.
There is also the debugger DB, which is a bit similer to GDB, and its upgraded "graphical" cousin SCD. They work with output from any of the toolchains as long as you've enabled debug symbol output. They're a little wobbly but they work pretty well.
All that being said; you're really going to have to learn Japanese or get really good at using OCR and Google Translate and figuring out what things mean. The tools and the best documentation are going to be difficult to use and understand if you can't manage the language somehow. Otherwise it'll be like using a hammer when you just need to walk across the street and buy a screwdriver at Home Depot
Go peep the X68000 docs at the Data Crystal
wiki and get a copy of Inside X68000
I have an HDS specifically set up for development that includes the 3 C toolchains and a heap of UNIX tools (like the DASH shell and typical UNIX things). I can probably put it somewhere if you want a copy.
I should probably properly write all this up someday. ~shrug~ If you have questions, ask. I don't promise to have an answer, but I might.