Add attribute to shadow dom element


I have the following problem with Firefox 68.7.0esr (32-Bit) and Win10 Enterprise (64-Bit) (no other browser tested because of company policy).

English is used for spell checking the content of my TextAreas, no matter what language is specified in the html tag of the page.
A solution would be to set the lang attribute in the shadow-dom of the TextArea again. However, I have not yet found a way to do this with pure Java.

Can anyone help here?