New: “Migrating from XeTeX to LuaTeX”

docs/_includes/menu.html

@@ -31,6 +31,7 @@
       <li><a href="#">Guides</a>
       <ul>
         <li><a href="https://latex3.github.io/babel/guides/which-method-for-which-language.html">Which method for which language</a></li>
+        <li><a href="https://latex3.github.io/babel/guides/migrating-xetex-luatex.html">Migrating from XeTeX to LuaTeX</a></li>
         <li><a href="https://latex3.github.io/babel/guides/non-standard-hyphenation-with-luatex.html">Non-standard hyphenation with luatex</a></li>
         <li><a href="https://latex3.github.io/babel/guides/using-babelprovide-to-modify-or-extend-locales.html">Using \babelprovide to modify or extend locales</a></li>
         <li><a href="https://latex3.github.io/babel/guides/useful-links-to-tex.stackexchange.html">Useful links to tex.strackexchange</a></li>

docs/guides/migrating-xetex-luatex.md

@@ -0,0 +1,49 @@
+# Migrating from XeTeX to LuaTeX
+
+**Draft**
+
+In many languages and scripts, particularly Latin, Greek and Cyrillic,
+a document written for `xetex` should work with `luatex` without
+changes.
+
+## Fonts
+
+By default, the font renderer in `xetex` is Harfbuzz (the only
+available). With `luatex` the default renderer is `Node`, which is not
+suited for some Asian scripts (Indic, South Asian). So, you may need to
+replace something like
+```tex
+\babelfont{rm}{FreeSerif}
+```
+with
+```
+\babelfont{rm}[Renderer=Harfbuzz]{FreeSerif}
+```
+
+## RTL scripts
+
+The `xetex` and `luatex` models are quite different. Actually, `xetex`
+lacked a true RTL model. The main change is to replace the package
+option `bidi=bidi` with `bidi=basic`. If you were using `bidi=default`,
+no change is necessary.
+
+Note `xetex` only ‘reverses’ the text, while `luatex` ‘reverses’ but
+also margins, columns, and so on, so with these elements expect
+different results.
+
+## `Mapping`
+
+This font feature is not available in `luatex`, but it can be replaced
+advantageously with
+[transforms](non-standard-hyphenation-with-luatex.html). There are
+already some transforms to transliterate a few languages. Native digits
+are already handled by `babel` out of the box.
+
+## ‘Interchar’ mechanism
+
+It can be replaced with transforms, too.
+
+The package `ucharclasses` can be replaced in most cases with the
+`babel` option `onchar`. The latter, unlike the former, works with RTL
+scripts.
+

docs/news/whats-new-in-babel-24.13.md

@@ -2,17 +2,20 @@
 
 **Draft**
 
-There is an important notice related to the LaTeX kernel, which affects
+There are important news related to the LaTeX kernel, which affects
 how to deal with languages. Now, according to [Engine news from the
 LaTeX Project](https://www.texdev.net/2024/11/05/engine-news-from-the-latex-project),
 “LuaTeX is the recommended engine for all new documents”. Furthermore,
 “it is time to move away from XeTeX: certainly for new documents, and
 even for existing ones”.
 
-There is some work in progress to provide short examples in many languages.
+[Here](migrating-xetex-luatex.html) is a document (still a draft) with
+some hints on how to migrate from `xetex` to `luatex`.
+
+Work is underway to provide short examples in many languages.
 See [Index of locale guides](https://latex3.github.io/babel/guides/index-locale.html)
 
-## Transform variables fo French
+## French: Transform variables
 
 When the `ini` file is used, you can now configure the spacing with
 punctuation, with the new transform variables. See [What’s new in
@@ -30,7 +33,7 @@ With, for example:
 spacing with the colon is slightly smaller (default is `.5`). Units
 are the current ordinary space.
 
-## Pashto Persian and Islamic calendar
+## Pashto: Persian and Islamic calendars
 
 Added strings for the those calendars in the locale for Pashto. Since
 the default calendar was Persian, an error was even raised (which is