Adding a language tag is always helpful to the search engines.
The search engines could figure out which language your site is using using a variety of factors, including the sites that link to your site--if they're in Spanish then your site might be in Spanish, as well.
Other factors that may be taken into account are where your site is hosted, the domain name's TLD, and the language meta tag--which, by the way, can also be specified in the < html > tag, as well (e.g., < html lang="en" >
There's more information about specifying the language
here on the W3.org site.