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What should I add for the other language which is say, Spanish?<html lang = "es"> of course. No need to get more specific ("es-es" and so on) unless you're offering more than one version of the same language.
<link rel="alternate" href="https://www.example.com" hreflang="en-us" />are not for your page with both languages. That tag is for alternative pages with one language or the other; that format is for alerting Google to the presence of substantially identical pages in another language. There's more at Google: [support.google.com...]
<html lang="en, es" />
<html lang = "en, es">Gosh, not2easy. It never occurred to me it was legal (as it were) to do this. But what does the poor screen reader do?
Google uses the visible content of your page to determine its language. We don’t use any code-level language information such as lang attributes, or the URL. You can help Google determine the language correctly by using a single language for content and navigation on each page, and by avoiding side-by-side translations.But that says you should have separate pages for each language. My guess is that if it is ONE page, it may not be a big deal. If it amounts to large chunks of the content, it would be more important.
not even this formatYes, yes, you can do that [w3.org] (or rather, you can still do it in HTML 5 [w3.org]) if you like--though the linking text should already make it obvious (“page in English”, “pagina-or-whatever-the-word-is en español”).