CSplain is a small extension of basic Plain TeX macros, the
formats csplain and pdfcsplain can be generated. It supports:
hyphenation of words for 50+ languages, simple and powerful
font loading system (various sizes of fonts), TeX, pdfTeX,
XeTeX and LuaTeX engines, math fonts simply loaded with full
amstex-like features, three internal encodings (IL2 for
Czech/Slovak languages, T1 for many languages with latin
alphabet and Unicode in new TeX engines), natural UTF-8 input
in pdfTeX using encTeX without any active characters, Czech and
Slovak special typesetting features. An important part of the
package is OPmac, which implements most of LaTeX's features
(sectioning, font selection, color, hyper reference and urls,
bibliography, index, toc, tables,etc.) by Plain TeX macros. The
OPmac macros can generate and bibliography without any external
program.

Homepage:
https://ctan.org/pkg/csplain
