The default human language of each page needs to be programmatically determined. Use, for example, the lang attribute in the <html> element. This helps screen readers read the content in the correct language. This might not seem like such a big deal but I recommend trying to understand the content of web pages when it's read in the wrong language! It is both horrible and very confusing.
If you have content on a page in another language, programmatically provide the language of that content. Remember that this includes any content in another language (apart from names and such), don't forget your language switcher menus. Example of language switcher with and without the language attribute.