How do I activate translation in MarketPress and SimpleMarket or FrameMarket theme?

Hello. I have translated MarketPress to Slovenian language (only the parts needed, other translations are still in English) and named it mp-si_SI with PoEdit and it's still in English when I install it. My WordPress is in Slovenian language. Also, I've translated THEMES: SimpleMarket and FrameMarket and neither of them show up in Slovenian language. How can I activate my language in the plugin and themes?

Thank you!