If you only need spellcheck, you can download a smaller "Proofing Tools Only" pack instead of the full Language Pack. Conclusion
For a full list of (the additional languages that have proofing tools inside a given language pack), Microsoft provides a comprehensive table on its Deploy Office portal.
Even with an offline installer, things go wrong. Here is the fix for the top 3 errors. Microsoft Office 2016 Language Pack Offline Installer
IT admins can use one file to update multiple computers via USB.
to create a local installation source that includes all required languages. Installation Steps (Offline) If you only need spellcheck, you can download
Before downloading, it is wise to check the "Extent of localization" for your chosen language on the Microsoft Language Accessory Pack page. Some packs are marked as "Partial," which means some parts of the Office interface may still display in your base Office language.
If you are using a version of Office 2016 (purchased through an enterprise agreement), you cannot use the standard Language Accessory Pack. Microsoft requires that you obtain language packs through the Volume Licensing Service Center (VLSC) . Here is the fix for the top 3 errors
Enables specific keyboard layouts corresponding to the language's unique characters.