Site language

If you use the correct language for your site the search engines will show your search result in favor to other results. Another aspect is that assistive technology like a screen reader will pronounce the text with the correct spoken language if you set the correct language tag. 

Language codes on W3 Schools

If you have a localized language, like french in the Netherlands or Switzerland you can also add in detailed information making your search result even better ranking for local search queries.

This article describes how to do localized language tags.

Language tag can also be applied globally as nl-be, Dutch for Belgium on the whole site and in content with french quotes you add the language tag for French for Belgium to that quote with nl-fr.