“Formal” can mean different things in different languages – respectful, cold and distant, official, plural, etc. So the answer often depends on the language – how the formal/informal registers are used – in addition to depending on who “we” are as an app.
To answer the first part, it can be valuable to look at a language-specific localization guide. I’ve found the publicly available ones from Microsoft useful – Microsoft Localization Style Guides - Globalization | Microsoft Learn . [Setting aside any other feelings I might have about following a Microsoft model for something 
– they’ve got a lot of experience in software localization!]
For Anki, the documentation and in-app localization should be approachable and not-overly-technical, but it should also be respectful. I can’t tell you whether that lands on fully-formal or kinda-formal language for Croatian.