To write your own language, you simply need to think of a syntax and program a parser which will translate it into executable code.
A few large programming languages which will go un-named simply take their own syntax and use their parser to translate it into C code, and then run it.
I considered making my own language, for fun, something along the lines of PHP BASIC. It'd basically allow you to do stuff like
Code:
For $i = 0 to 10
If $i % 2
echo ($i . "<br />")
End If
Next
Seems like a fun project huh?
Regards,
Patrick