You shouldn't edit the .yml
files directly, because future updates will overwrite any changes you make.
The recommended way is to create an alternate English language pack with your name as the vendor; that is, instead of flarum/flarum-ext-english
you want to create an extension called (for example) menchold/flarum-ext-english
. Modify that, install it, select it as the default language for your site, and then uninstall the bundled English pack, so that any changes to that will not overwrite your modifications.