A lexer has to be added as a built-in filetype. "Somebody" has to make a pull request doing that, but modula is pretty much a historical language and this is the only request that github has, and the mailing list archives have no references to modula either, so not sure how likely it is that the PR will be accepted.

See for example https://github.com/geany/geany/commit/90c6096ed6ea167f9100ce8f74229a3f47acc2... which was adding Julia, see #2584. That language has a ctags parser that doesn't exist for modula, so it will be simpler but much will still be needed. See also HACKING although its getting a bit outdated.