Language = , does one override the other?

Let us say we have a website that is 95% in standard English.

Let us also say that of those 5% of pages that are not in English, that we set a meta language tag to the appropriate language, including those pages written for different types of English such as en-US, en-GB, en-AU, etc.

Would the meta tags on the individual pages provide the proper language for search engines, browsers, screen readers, etc?
For example, we modify the site to always print lang=en, but a page written in Spanish has a meta-lang tag with the appropriate code. Would this be proper? Functional? Problematic somehow?