You can now install Crystal from Termux’s official package repository on AArch64 Android devices:
u0_a211@localhost:~$ apt search crystal$
Sorting... Done
Full Text Search... Done
crystal/stable 1.9.0 aarch64
Fast and statically typed, compiled language with Ruby-like syntax
u0_a211@localhost:~$ apt info crystal
Package: crystal
Version: 1.9.0
Maintainer: @HertzDevil
Installed-Size: 23.9 MB
Depends: libc++, libevent, libgc, libgmp, libiconv, libllvm (<< 17), libxml2, libyaml, openssl, pcre2, zlib
Recommends: clang, libffi, make, pkg-config
Homepage: https://crystal-lang.org
Download-Size: 3590 kB
APT-Sources: https://packages.termux.dev/apt/termux-main stable/main aarch64 Packages
Description: Fast and statically typed, compiled language with Ruby-like syntax
The playground and all third-party dependencies in the standard library are available. The interpreter is currently not built. Note that -Duse_libiconv is not enabled by default at the moment, and without that flag you will end up linking against the far less useful system iconv in Bionic.
Crystal can be installed with just pkg install crystal. If you have Docker, you could also clone the build environment and try building Crystal locally with scripts/run-docker.sh scripts/build-package.sh -I -a aarch64 crystal.