As a German you never know when you might need to brush up on your Bulgarian, the same goes the other way around too, and while there are plenty of translators out there, they require constant Internet connection and are mostly limited to just definitions and straightforward translations.
PONS Dictionary German - Bulgarian Concise is a great tool for any German wanting to learn Bulgarian and vice-versa.
The app is part of a software suite that comprises multiple language vocabularies, and each time you install one, you just get that version library added to the main program.
For example, if you already have all the other programs from the PONS series made by this developer, installing this one will just add the German-Bulgarian dictionaries in PONS' library.
The app is straightforward and easy-to-use, since the UI is minimalistic and the icons are self-explanatory and uncluttered, so you know what you have to do no matter if you have previous experiences using this app or not.
There are multiple ways in which you as a German can learn Bulgarian and vice-versa, starting from plain-old translations and definitions, to mini-games such as wild-card search and graded quizzes.
All you have to do is select the words you want to practice and add them to the list of words you want to keep for later reviews. Words that are a bit trickier to pronounce are also accompanied by a recording of proper pronunciation so that you won't end up learning things the wrong way.
If you want to cut to the chase and find irregular verbs (a nightmare when learning new languages), then you have a complete list of all the verbs from that language.
If you're a German wanting to see the famous Bulgarian roses or a Bulgarian that wants to go to Oktoberfest, then PONS Dictionary German - Bulgarian Concise is an app that you will truly find useful.