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Is the forum js file ever overwritten?
There is a high chance that this happens every time the software is updated.
I'm just editing some of the default wording of Flarum (ex: new topic button).
To change the wording of Flarum elements, you need to edit the .yml file of your language extension. This file may also be overwritten during future updates, but at least that shouldn't break your forum. It would simply undo your changes, which you would have to reapply after an update of the language extension.