Product group in feed header data is a single level group id or name.
To import a name value for product group you need to download the import template first. Then add values to the list.
The template supports import to the languages you have defined in feed, so when you click on download template you need to select the languages you want to use when updating or creating new "product groups".
Make sure you create new values on all languages so that users on different languages setting than yourself will see the value in their language as well. This is also important if you will be using these in integrations
Blank on a language will result in blank value for that language.
If you just "update", you can update only on the languages that are missing a value for a product group.
You need to specify an import code when importing product group.
To see the import codes that are in use on "Product group", go to "Basic data". Under Product, click on "Product group". Here you will se a list of "product groups" (you can also export this list if needed).
Blank on import codes are manually created Product groups that has not received import codes on creation. These cannot be updated in an Excel import (unless they are given an import code first).