"...converting your HTML or XHTML templates..."
I would use the word 'documents' or 'files' instead of 'templates'.
".. appropriate Unicode encoding. Yo will need to plan ..."
" .... Internal data sorces ..."
"Ensure that your server does the right thing; check HTTP Response header."
Perhaps, rephrase to:
"Ensure that your server's HTTP Content-type response header does not contain a different encoding to that declared in your document"
"Marking up the primary language of a document and any change of language of a document is good internationalization proactice..."
Suggest it be changed to:
"Marking up the primary language of a document, and any change of language within a document, is good internationalization practice..."