What is the implication of the warning in the fontspec manual with respect to babel and LuaTeX?

Question

In the fontspec manual, there is a section on LuaTeX that states:

babel   The babel package is not really supported! Especially Vietnamese, Greek, and Hebrew at least might not work correctly, as far as I can tell. There’s a better chance with Cyrillic and Latin-based languages, however—fontspec ensures at least that fonts should load correctly, but hyphenation and other matters aren’t guaranteed. Under XeTeX, the polyglossia package is recommended instead as a modern replacement for babel.

What are the technical implications of this? Should I be afraid of anything?

Answer 1

Languages that set an output encoding as part of their working in babel will have problems. For example, \selectlanguage{vietnamese} issues

\fontencoding{T5}\selectfont
\def\encodingdefault{T5}
\language\l@vietnamese

and of course the first two lines are nonsense with fontspec and OTF fonts. The same problem along with others will appear with languages written in other scripts (Cyrillic, Greek, Hebrew). See, for example, Table of contents encoding issue with LuaLaTeX.