Different language of wordpress and content

Hey, so i decided to start a blog finally few months ago and at first i wanted the blog to be in english, but then i changed my mind and decided to do it in my native language which is slovak. I’m worried how does this affect SEO? does it really matter what is specified inside the lang tag?
Or is page ranking based on content?

I downloaded a plugin which actually changed the language defined in html to my native language, even though wp itself is still in English which is great, and I’d like to keeep it that way… So yeah… does this affect SEO or not?

Thanks 🙂