A: Yes. Go to Windows Settings > Apps > Apps & features. Find "AutoCAD 2021 Language Pack - [Language]" and uninstall. This will not break your base English installation.
A: Yes, Autodesk provides Language Packs for AutoCAD LT 2021 as well. The process is identical. Autocad Language Pack 2021
A: No. The drawing file (DWG) is binary and independent of the UI language. A drawing created in English AutoCAD will open perfectly in Japanese AutoCAD. Only the interface changes, not the geometry or text entities. A: Yes