HTML lang when optimizing for language?

[TomSnow]

Client has a Spanish site in the US.

HTML lang is set to en=us.

Google says they ignore HTML lang tags anyway. But I'm wondering...

Should I have my Spanish site set to lang=es instead of en=us? Or is there some other way of optimizing for my language or region?

PS 50% of users have browsers set to en=us while the other half are es.

[lucy24]

Oh, ffs. Sorry.

What language is at least 51% of the visible text content on the page? What language should visually impaired users’ text readers use?