There are a number of misconceived language treatments in source code:

• a non-dominating, uncommon language code mul introduced in 6ea69c5b3a2cb73ccacd8b08affbbd10172ceb33 (?)
• no clear overriding attributes on various localized elements, .central-textlogo-wrapper for example

Although mul is a valid language code for multiple languages, it should probably not be used and rather the dominant language should be set on the root element.

See

• https://stackoverflow.com/questions/7076837/what-html-lang-attribute-value-should-i-use-for-a-mixed-language-page