I've used extensions like Toucan before, which basically changes a few words of any article you're browsing on a website, but never came across Language Reactor, and this seems like a fun tool! I primarily watch films on MUBI, but got myself a Netflix subscription just because of French tv shows. Might just give this extension a try. I'm surprised it works, Netflix being so guarded against piracy, I can't even take screenshots.

because a long-running tv show will use more of the same vocabulary

Agreed with TV series having words with repetitive qualities, also I feel like Tv shows use more colloquial and applicable language compared to cinema. More trendy and street-friendly words. Great for slang words too.

When you watch Netflix, which language it's set by default for you? English or Mandarin? Do you hover over unknown Mandarin words and find out the English translation?