Unknown language in Word and Excel

The behavior is caused by additional Office language components being installed so that text in documents is tagged and proofed in those languages (Afrikaans in this case) instead of English. Language is a character-level attribute in Word and Excel, so once content is marked as Afrikaans, spell check and some UI elements will follow that language until corrected.

Uninstalling the extra Microsoft 365 language builds, as already done, is the correct way to stop Word from constantly switching dictionaries due to multiple full language installations. To fully clean up and prevent recurrence:

1. Confirm only English Office language is installed
   In Windows Settings → Apps → Installed apps, ensure there is only one Microsoft 365 installation and that no separate Office language packs (Afrikaans or others) are installed. Removing extra language packs prevents Office from automatically switching to those languages.
2. Set English as the default editing and proofing language in Office
   In any Office app (Word or Excel):
   1. Go to File → Options → Language.
   2. Under Choose Editing Languages, select English (United States) (or preferred English) and click Set as Default if it is not already.
   3. If other editing languages are listed that are not needed (such as Afrikaans), remove them from the list.
   4. Under Proofing, verify English shows as Installed. If another language is installed and not needed, remove its language accessory pack via Apps → Installed apps.
   This ensures new documents default to English and that Office does not try to proof in Afrikaans.
3. Fix existing documents that show Afrikaans text
   Because language is a character-level attribute, existing documents may still be tagged as Afrikaans:
   1. Open the document in Word.
   2. Press Ctrl+A to select all content.
   3. Go to Review → Language → Set Proofing Language.
   4. Choose English (United States) (or preferred English).
   5. Clear Detect language automatically so Word does not re-tag text based on content.
   6. Click OK.
   For documents that still misbehave, modify the Styles (e.g., Normal, Heading 1) to use English as the language so future content typed with those styles stays in English.
4. Prevent future automatic language switching
   • Keep only the required Office language installed (English).
   • In Word, ensure Detect language automatically is turned off in the Set Proofing Language dialog.
   • Avoid installing additional Office language packs unless they are truly needed. If a future update adds languages again, repeat the uninstall and language-preference steps.

This combination—removing extra language packs, setting English as default in Office language preferences, and correcting the language attribute in existing documents—prevents Word and Excel from switching to Afrikaans in the future.

References:

• Change the language Office uses in its menus and proofing tools
• Language Accessory Pack for Microsoft 365
• Microsoft Word (365) remains extremely slow - Microsoft Q&A
• how to stop language defaulting to us english - Microsoft Q&A
• The default language - English - keeps switching to Portuguese even after RE-setting default to English - Microsoft Q&A