The babel package for supporting multiple languages by redefining various internal bits of text to replace, eg. “Chapter” by “Kapital” and by defining various shorthand mechanisms to make it easy to type the extra non-latin characters and glyphs used by those languages. Each supported language or dialect has a module which is loaded to provide the needed definitions.
To the extent: that LaTeXML’s input and output encoding handling is sufficient; that its processing of raw TeX is good enough; and that it proceeds through the appropriate LaTeX internals, LaTeXML should be able to support babel and arbitrary languages by reading in the raw TeX implementation of the language module from the TeX distribution itself.
At least, that is the strategy that we use.