The Languages files in Garden Planner have been recently updated. Polish, Russian and Norwegian have been added to the languages files as well as updates to the existing translations.
However, please note that only the Plants and Objects names gets translated, the rest is still in English. But this is still useful if you only speak a little English or if you speak English well but are developing plans for someone who speaks a different language.
Furthermore there is now a simple tool that lets you update the languages files yourself if you notice any incorrect translations. The details on the languages tool are here.