I can see where your confusion comes from, the interface is indeed not very clear. It suggests that there is something like a ‘device language’ that can be different from the ‘app language’. However, there is no such distinction under the bonnet.
Language support on iOS works like this. Every app has a number of folders for language-specific assets, named after a language code. For instance, an app can have a folder called en.lproj/ as well as de.lproj/ and ru.lproj/ and so forth. When the app is loaded, it will load the assets from one of these folders, based on your preference. This is what determines the presented language of the app. This mechanism isn’t just used for apps, but for system components too, such as the lock and home screen. It is also used by preinstalled applications, such as Settings. The underlying functionality is the same. There is no such thing as a ‘device language’ in this technical sense, there is only this list of languages that iOS uses to determine which language assets to look for.
The ‘device language’ is however a set of languages that Apple themselves provides complete translations for, including for system applications and the lock/home screen. As long as you have any one of these languages on your list, system apps and lock/home screen will be properly translated. There are many more languages available that Apple doesn’t provide translations for, but which third-party developers can support. This is where Apple distinguishes between a primary ‘device language’ and a secondary ‘app language’.
For instance, Apple does not support Irish Gaelic as a primary language (or ‘device language’), but it does provide limited support for it, notably for dates. You can move Irish Gaelic to the top of the list and it will supersede English in some cases, but for the most part, because the translation is not complete, you will only see English, because that is still the device language. In your case, because English, German and Russian are all languages that Apple provides complete translations for, they will compete to be the ‘device language’. Whichever one is nearest to the top of the list will be used for system apps and lock/home screen.
iOS has decent localisation support and Apple does ship a fair number of translations. However, this isn’t what I understand as multilingual support, i.e., supporting multiple languages at the same time.
I speak 5 languages, most of them on a near-daily basis. I would love to be able to speak to Siri in any language I want without having to change my settings, type in any language in any app and still get suggestions without having to switch manually. I want iOS to be more context-/language-aware and adapt to whomever I am talking in iMessage, the language I am reading on a website, the place names I am receiving directions for in Maps. iOS is still principally based on one language at a time, it just is a tiny bit smarter now and recognises certain languages when I am typing them.