The language metatag is used to indicate which natural language your website is in (e.g. English, French, etc.) as opposed to the coding language (such as html).
If your website is all in one language, then search engines will easily be able to identify which language it is in and therefore this metatag is not necessary. If your website is written in more than one language (e.g. French and English), then it's best to include this metatag on each page to tell search engines which language that specific page is written in.
Here is a list of commonly spoken languages and their code for this meta tag:
To use this metatag, enter the following html below the first <head> tag and above the </head> tag, replacing the XX with the desired language code.
<META HTTP-EQUIV="Content-Language" content="XX">
So, for example, to signal that the web page is in English, you would enter the following:
<META HTTP-EQUIV="Content-Language" content="EN">