Our team is fixing all WCAG guideline bugs in our product. The 3.1.1 guideline can't be met because the Kendo editor doesn't allow for setting a lang attribute on the iframe document, nor does the editor infer the language from the parent document and add that lang attribute.
Open the editor example page (https://demos.telerik.com/kendo-ui/editor/index). Note that the editor iframe document contains <html> at the top but does not contain a lang="en" attribute within it. Further, edit the example on the page and add lang="en" to the <html> attribute at the top of the example. Run the example. The iframe still does not contain the lang="en" attribute.
Include the lang attribute on the child document when the parent document contains one or make a setting available in the configuration JSON for the editor to include a lang attribute in the child document when an iframe is used.