When I switch application language to Japanese, I can't enable inherit alpha, alpha locked and onion skins on Timeline docker and layer docker. It is the same state when I switch other language but American English (then everything goes smooth).

Hmm, I think I could switch onion skins on Timeline docker using Japanese UI... (I can't test it again until tomorrow night Japan time, though)

I wonder if deleting kritarc would make a difference, I'm not sure if you saw this updated part of the announcement. (I added translation of this part to your translated article, btw. Hope you do not mind.)
https://krita.org/item/krita-2-9-animat ... -released/

For people who previously installed Krita, to get Instant Preview to show up on the view menu, delete the krita.rc(not kritarc) file in your resource folder(which can be accessed quickly via Settings->Manage Resources->Open Resource Folder) and restart Krita. Or just use the hotkey Shift+L.

Well... Checking to delete krita.rc from /share/apps, I wasn't able to start krita 2.9 ae. So I think the way shouldn't be recomended, especially as official announce..

If you delete both krita.rc under user/... and krita_2.9.9.2ae_beta_.../..., krita 2.9 ae beta won't start. This should be taken care of.

And, I used krita 2.9 ae beta with .mo files I edited. When I use one with vanila mo files, I can switch onion skins but inherit alpha and alpha locked. I think this is because the latter one's .mo files lack translation about onion skins

Concerned hearing your krita.rc experience(deleting krita.rc in your %APPDATA% should not fail Krita's launch). Need to test this scenario again with the original package and the replaced package(with bumped version).

Also I did not specifically touched Alpha buttons - so it might be issues with alpha & alpha locked. This definately sounds like a bug and should be reported in bug tracker.

https://bugs.kde.org/show_bug.cgi?id=355913

Repro Inherit Alpha/Lock Alpha issue(and added comment to bug tracker)

As for krita.rc, I double checked and observed no crash in deleting krita.rc in my configuration folder C:\Users\<your name>\AppData\Roaming\krita\share\apps\krita\krita.rc.
This is different from krita_2.9.9.2ae_beta_x64\share\apps\krita\krita.rc. If you delete this file which is a part of the package, Krita will surely crash. This is expected. That's why the announcement specifically says "delete the krita.rc(not kritarc) file in your resource folder" not "in your package"
So I think there is no Krita error here.