Whose follow-up? Did watcher's ever do anything but refund its cost? That excludes outdated translations. Sounds like a marisa card too, which hasn't gotten updated in a year.

In my game it says "Play Follow up and follow up 2 will be added to your hand".

Quick google says it should read, "play after another attack and gain 1 energy".. which it actuall does do.

It is a Watcher Card, I dont use Marisa

I suppose you are using an outdated translation after all. You could be the best chad and poke the devs on discord to offer updating that language's translation yourself.

Seems it is a bug added by the Servant Mod. Someone else mentioned it on the mod page last week it seems:)

Also, im Australian, and I think the game is made in English:)

Good job conducting the research yourself. Learned an oddity about mods today myself, and that I should ask more questions before making assumptions based on similar threads.

If you want a technical explanation:

The way the game loads localisation text is through a json file, using a key to lookup data.
It's formatted in this way:

• KEY_NAME : (The name used by the game to lookup from the json
  
• NAME: [Text] - Used for the name of the card
  
• DESCRIPTION: [Text] - Used for the card's text

Of course, some cards have extra fields, for example Blizzard has an extra field to display how much damage it does on the card.

Now, the problem here is when localisation from a mod adds a card with the same localisation key. Mod localisation is loaded after the basegame, so whatever the mod adds takes precedent over overwritten assets.

So, because The Servant has a card called FollowUp, the localisation files override basegame's followup with the text of the modded equivalent.

The way current mods circumvent this issue is to prefix their own modID (unique identifier) to the localisation files, so the card would become (MODID):FollowUp

But the servant seems to be a very old mod, and it doesn't use this system for localisation.

TLDR: ModIDs normally prefix their assets so it doesn't override basegame text, Servant mod is old enough that it doesn't do this, so two localisation keys are identical, and the modded one has priority.