Search Engines detect the language in which your website has been written, which increases its regional importance as well helps it understand the text.
Therefore, it is a must to incorporate language tag and you must make sure you that Google detects your webpage language the same as specified by you.
Furthermore, when users translate your webpage to any other language, a source language makes it easy for web translators.
Dublin Core is used to provide metadata for resource discovery.
However, make sure the designer adds the tag on all page as the Doc Type is responsible for similar representation of your webpage on all browser.
The Doc Type of your webpage does not have any influence on your Search ranking.In terms of page Title, H1 heading, or generally including all the keywords your users will focus on is a great practice.