→ As far I know,
Smith Micro's Manga Studio 4 brushes (.brp files) are not compatible with Celsys' ones (.tos files), they don't have the same file extension.
I don't know if this will happen with the new
Manga Studio 5 version though, but I guess this time all brushes could be .tos files, I guess brushes will finally have the same file extension in both versions.
→
I wonder if you also can download Clip Studio Paint materials from the Celsys website with Manga Studio 5, since you ONLY could do it via "ClipStudio.exe" start menu (you know, that menu to update and launch the programs and download/organize materials from Celsys website):

→
Will you be able to download Celsys' materials from the Smith Micro's start menu too? (I doubt so, since Smith Micro and Celsys are two different companies)
→
Smith Micro (the distribution company) didn't update Manga Studio EX 4 since 2009 whereas Celsys (the product developer) is updating Comic Studio EX 4 and all their programs CONSTANTLY.If they are going to do the SAME with Manga Studio 5... I would prefer the japanese version to be honest, since this program is Celsys' CREATION (and cheaper) and they are constantly updating. It wouldn't surprise me to see Smith Micro launching only a couple of updates in 2013-14 and later nothing else until... 2020??? (and I guess they would directly launch another new translated program... like they have been doing until now)
Moreover,
Celsys' Clip Studio Paint EX will be constantly updating and releasing new tools until Autum/Winter 2013 while Smith Micro is going to launch an incomplete official EX version in Summer 2013. Smith Micro is just waiting for Celsys' updates before launching versions and updates (plus, as I said before, they don't usually update as often as Celsys do), Smith Micro NEEDS to go almost 9 months or 1 year behind because they are not the devepolers of this program, but Celsys.
→ Regarding the translation,
I hope Smith Micro releases a MS 5 trial soon, so we only need to get those 2-3 files inside the 'resources/other' folder to have a full english translation for our CPS.; or well, I just hope it could work.
→ Welp, at least we could get this to understand better how the japanese version works, since both versions are similar:
1. User's Guide for Manga Studio 5
2. Tool Guide for Manga Studio 5→ As BrianK said above:
I hope that english translation for Celsys' Clip Studio Paint versions/updates continues! 
These programs are Celsys' own creations and this company is deeply thrown into them (and are constantly updating, unlike Smith Micro).