I am also not sure for Croatian and other languages that have kupite instead of kupi. In slovenian kupi would be better suited for slogans or brands since it is used for a single person. But I am not sure since some languages are only similar but I do not speak Croatian or other languages. However in Slovenian to put something on a panel on a protest I would use “kupujte evropsko” meaning something like “you all should buy european”.

However all versions can be used in all mentioned occasions. I just mentioned what would be better suited for what occasion.

Slovenian translation is awful and has no meaning. It would be translated back to english something like: “Slow euroajaian”.

Correct translation would be:

1. Kupuj evropsko (buy european in general/every day)
2. Kupite evropsko (buy european now eg. For this product)
3. Kupi evropsko (more personal - you specifically buy european now eg for this product)

I vote for 3. If it a label/brand/badge on a product. And 1. If this is US boycott slogan.