@AlexWright thank you so much for this post! It helped me make the IDENTICAL fURLs I had customized back in the day on 3.4.x to be the same in 4.4!
BTW my default app will be Pages.
What you did not mention that you have to translate is
"topLevel": "forums",
to your language. Thus I was able to translate this toplevel fURL and translated the rest in ACP and in the end I got results such as:
http://website.com/forum/f1/forum-category-name/ http://website.com/forum/t46405/topic-name/
Basically you have to do this for every app you want to translate, go to its folder, find furl.json file and edit it.
Today IPS support told me this can't be done, but there it is. 🙂