It helps to set the language of a page when you're developing to meet WCAG 2.0 standards. Specifically, the 3.1.1 - Language of Page rule.
You can accomplish this with the lang attribute on the html tag, like so:
If you would like to set a specific piece of content to a different language, use the lang attribute there as well. This will help screen readers pronounce the content correctly.
For example, a link to the Japanese version of a website would be pronounced "Nihongo", not "Ideographic Character Ideographic Character Ideographic Character".
<a href="#" lang="ja">日本語</a>