Page doesn't say what language it's in
Screen readers need to know what language a page is written in before they can read it aloud correctly. Usually a one-line theme fix.
What a shopper experiences
Every page carries a hidden setting saying which language it is written in. Without it, a screen reader falls back to whatever the shopper has configured. An English store can end up read aloud using Spanish or German pronunciation rules, which is close to unintelligible — the words are right and nothing comes out understandable.
Why it shows up
This is set once in your theme and applies to your whole store, so it is all-or-nothing. Current Shopify themes handle it already. Seeing this usually means an older theme, or one that has been heavily customised at some point.
How to fix it
It is a single attribute on the page layout, and your theme developer will recognise it the moment you mention it. If you do not have a developer, Shopify support can point at the right line, or updating to a current version of your theme usually resolves it. Either way it is a small job — it is on this list because the impact is large, not because the fix is hard.