AdmiralJuicy have you tried?
A subset of HTML should be supported, but it's very limited. It's possible only 1 of each tag can be used in a string, and you cannot provide any HTML attributes. It's also possible self-closing tags are not supported. So you're left with mostly code, strong, em, sub, sup. Links a are supported but the link URL must be provided by the code, so you can only use it in your translation if the original also had one.
For more advanced content injection, if there's no option in the UI or in a third-party extension, you should use a custom Flarum extension and the javascript API.
