What language is used to build low level languages like c++ and java?
How could you build the first language with no language?
Typically another machine or another language is used to write the first assembler and the first compiler.
As long as a working computer and a working language is available, even if rather different, the problem can be solved in two steps.
Write target language x for computer y in language z on computer (urk) a.
Write target language x for computer y in language x. Now a single compile on a will produce a translator that can run on y, and the second compile can then be on y with a fully-bootstrapped system.
The problem becomes simpler if the languages or machines don't differ.
Bootstrapping can also be done incrementally, and perhaps this was more common 50 years ago.
Something like Forth might make a good intermediate step.