Yes, it is. NumXL has users across the globe with various non-English excel installations (e.g. French, Spanish, German, Italian, Swedish, Serbian, Korean, Japanese, etc.).
The only downside is the menus, user-messages and cells’ comments are either in English or Spanish depending on the Excel's language, and English is the default language.
Anyone who may be experience a locality-related (suspect) issue is strongly encouraged to contact our support. Those issues are given high priority, and a hot-fix (if any) is released with 2-business days.