Yes according to the World Wide Web Consortium (W3C) documentation you can include the main language for your document and declare another language for a specific element of the page. Here is the documentation. So the example you gave would be correct.
<html lang="en-US">
<p lang="hi">नमस्ते</p>
However, if you have SEO in mind and whatwant to rank that page for Hindi-speaking users, you might have problems to rank high as other competing pages might have better signals (title of the page, headers, internal links, external links) to rank for those Hindi terms.