Yes, Lewanski can only be Polish. Any Slavic language that would have an "sky" ending would use a "v" instead of a "w" and thus the name would be Levansky which could be Slovak, Czech, Ukrainian, Russian or Jewish. It is probably unlikely to be a South Slavic name. Anyway, the series creators used Lewanski, not…

The last name is Lewanski not Lewansky. There is a significant difference.