Language
SC 3.1.1 - What does "Language of Page" mean?
A screen reader pronounces your text using the language the page declares. Put the wrong one there and an English voice reads your Dutch page out loud. Not translated, just pronounced with English rules, which produces something close to noise.
SC 3.1.2 - What does "Language of Parts" mean?
Most pages are written in one language, with the occasional passage in another: a quote, a product name, a phrase the industry never bothered to translate. If you do not say where the language changes, a screen reader keeps applying the wrong pronunciation rules and the passage comes out as noise.