MangadexTitleTL от aerosock

This extension replaces the original titles in the DOM with the altTitles(names of the manga in other languages) from the API response that is used to populate the page.

The replaced titles are highlighted with a different color. In the extension settings you can change the color of the highlighting, change the desired title language, and turn the extension ON and OFF.

The extension doesn’t actually "translate" anything, it just substitutes the data with something that already exists on the site.

If a title doesn't exist in your language of choice it will default to English, or if English doesn't exist, it will leave the title unchanged