HTML meta tag for content language
<meta name="language" content="Spanish">
This isn't defined in any specification (including the HTML5 draft)
<meta http-equiv="content-language" content="es">
This is a poor man's version of a real HTTP header and should really be expressed in the headers. For example:
Content-language: es
Content-type: text/html;charset=UTF-8
It says that the document is intended for Spanish language speakers (it doesn't, however mean the document is written in Spanish; it could, for example, be written in English as part of a language course for Spanish speakers).
From the spec:
The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this might not be equivalent to all the languages used within the entity-body.
If you want to state that a document is written in Spanish then use:
<html lang="es">
You asked for differences, but you can’t quite compare those two.
Note that <meta http-equiv="content-language" content="es"> is obsolete and removed in HTML5. It was used to specify “a document-wide default language”, with its http-equiv attribute making it a pragma directive (which simulates an HTTP response header like Content-Language that hasn’t been sent from the server, since it cannot override a real one).
Regarding <meta name="language" content="Spanish">, you hardly find any reliable information. It’s non-standard and was probably invented as a SEO makeshift.
However, the HTML5 W3C Recommendation encourages authors to use the lang attribute on html root elements (attribute values must be valid BCP 47 language tags):
<!DOCTYPE html>
<html lang="es-ES">
<head>
…
Anyway, if you want to specify the content language to instruct search engine robots, you should consider this quote from Google Search Console Help on multilingual sites:
Google uses only the visible content of your page to determine its language. We don’t use any code-level language information such as
langattributes.
Google recommends to use hreflang, read more info
Examples:
<link rel="alternate" href="http://example.com/en-ie" hreflang="en-ie" />
<link rel="alternate" href="http://example.com/en-ca" hreflang="en-ca" />
<link rel="alternate" href="http://example.com/en-au" hreflang="en-au" />
<link rel="alternate" href="http://example.com/en" hreflang="en" />