dictionary configurer автор Siarhei Kuzeyeu

Warning
This extension change some attributes on pages to rule dictionary configurations, It can break some sites that depend on changed attributes.
The extension is too young and it possibly has some bugs.

A dictionary configuration that is used for pages manually can overwrite the extension configuration. If that doesn't work, you can manually clear all content preferences by pressing CTRL + SHIFT + DELETE (some sites can block this hot key) or through Firefox menu on about:preferences#privacy page in Cookies and Site Data menu item.

Similar extensions
1. Automatic Spelling Language

Known issues
1. It can break CSS that depends on changed attributes like [lang="en-US"]{...}. See for an example about the "lang" attribute here
2. It can break a JavaScript that depends on a lang attribute value.
3. main.ts file has a dirty hack to disable console.debug, It needs to be replaced by angular builders in angular.json