Greasy Fork is available in English.
Automatically highlight user-defined text with Seek function (2019-09-22)
< Feedback on Text Highlight and Seek
Well, not so much with Reddit.
Works for me on Reddit-- I just installed this to highlight certain users' names.
Are you using old or new Reddit style? I'm using old, i.e. https://old.reddit.com/
Still works great!