So, as I just found this via Google, I'll dig out this thread to help others:
You have to edit site/modules/InputfieldCKEditor/config.js and add the following section inside the CKEDITOR.editorConfig function:
const languages = {
1010: "de_DE",
1012: "en_US",
};
const t = setInterval(() => {
const ed = Object.values(CKEDITOR.instances).filter(x => x.config === config)[0];
const lang = ed.element.$.parentElement.getAttribute("data-language");
if (!lang || !languages[lang]) return clearInterval(t);
if (!ed.scayt) return; // wait until scayt is initialized
clearInterval(t);
ed.scayt.setLang(languages[lang]);
}, 100);
You can find out the language IDs by looking at the URL of your language configuration page - it ends with e.g. ?id=1012
