Searching Google for "Office 2021 proofing tools offline installer best" will lead you to third-party sites like MajorGeeks, Softpedia, or random GitHub repos. Be extremely careful.

Microsoft’s default delivery method for Office 2021 is the "Click-to-Run" (C2R) streaming installation. When you try to add a new proofing language via Settings > Time & Language > Language, Microsoft Office will automatically download the language pack from the internet.

This creates several problems:

The solution? The Office 2021 Proofing Tools Offline Installer—a standalone executable (.exe) or image file (.iso) that installs the tools without phoning home.

If you have a Volume License agreement (even for just 5 seats), log into the Volume Licensing Service Center.