Language undeclared - epub-lang
why pronunciation breaks, and the one-line fix
If a content document does not declare its language, a screen reader falls back to the reader's system voice. An English book read with a German voice is not a small annoyance; it is unlistenable.
The hand-fix
Declare the language twice on the root element of every XHTML file: the XML attribute and the HTML one. Then make sure the package document has a dc:language element.
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="en" xml:lang="en">
Passages in another language
Mark them inline, so the voice switches for the quotation and switches back: <span lang="fr">déjà vu</span>.
What Reflow does
Takes the language from the book's own package metadata, never guesses it, and writes it onto every document that is missing it. If the package itself declares nothing, the finding stays open for a human, because inventing a language is worse than leaving it blank.