It would be good practice to use the lang attribute to describe which language is used: http://www.w3.org/TR/REC-html40/struct/dirlang.html
I would define the language inside the chapter and, if necessary, locally inside the document.
You do not specify what type of doctype you use, but if you use XHTML, then there are also xml lang attributes:
http://www.w3schools.com/Xhtml/xhtml_syntax.asp
I don't know if this is “best practice,” but when I recently worked on an English and Arabic site, it was useful for me to use CSS classes to install rtl and ltr.
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