I have not done a test on XenForo to see what % of our content translates ( i'd have to buy a license ), but i imagine the % is very high because it appears to be a very refined platform.
Things that i am very sure won't translate:
1) captions on uploaded images
2) the ability to change the thread title after posting
3) the ability to change the thread title on a per-post basis
4) 100% of the backend settings
5) Those little 'post icons' for threads/posts.
Things that might be lost in the translation, but might be able to be reconstituted:
1) PMs - if these cannot be translated then I will come up with a way to archive and review them, since this forum has 14 years of those!
2) Avatars
3) Signatures
4) User profile fields such as location, your website, etc..
5) Your user ranking, IE whether you are a AAA battery or basically the sun
What i know would HAVE to be reconstituted and CAN be reconstituted:
1) Our custom smilies - fixable with some database/regex-fu
2) Youtube embeds - also database/regex-fu
3) Special formatting/BBCode - do i get to translate this? hopefully not!
XenForo is very promising and i am in the early stages of investigating it. If it lives up to the hype, it's a slam dunk for what will become endless sphere 2.0. I'm quite excited about it and will be taking a crack at it soon. If things go well, there will be a lot of exciting things in store for 2021.

"Love and compassion are necessities, not luxuries. Without them, humanity cannot survive." - Dalai Lama
My first major build: 1.6kW 8T MAC motor on a Trek 4500 MTB.
Monster MTB: Leafmotor 1500w @ 4kW on a Turner O2 full suspension.
The monster scooter: Cannondale semi recumbent - under construction.
Blue Dream: Maxaraya FS semi recumbent and high efficiency mid-drive - under construction.