Inventool works with Unity's localization package. You should already have it installed, as it's installed with Inventool itself.
Setup Steps
1. Add Locales
Locales must be added through Unity's localization package. Familiarize yourself with this package before moving on.
2. Enable Localization
You must enable localization on Inventool's end through its Settings.
3. Update Locales
This step must be repeated every time you add or remove locales through Unity's localization package. If your locales never change, you should only have to do it once after setting up your locales.
From Settings, under Localization, click Update Locales.
At this point, your localization options will be available for each Inventool object, and the displayed in-game text will update as you change the active locale through Unity's localization system.