When changing back and forth between languages in the Office applications the language that the add-ins are displayed in does not change.
Consider this scenarion: The Office base language is English with a French Language pack. The user can change back and forth between French and English for the Excel application but the add-ins will still be displayed in French.
Excel add-ins will always appear in the language that was installed last. Office Language changes have no effect on the add-ins.
1. Directly modify the Windows Installer metadata to ensure the Excel Add-Ins language matches the UI
2. Install the English base language with the Add-Ins. Install each additional language with a custom.xml file that excludes the Excel Add-Ins for that language
This is a known issue with the products listed in the "Applies To" section.