Never limit your career to only Nigeria there are a lot of opportunities out there in several sectors.

You searched the Job online.
They do a phone interview with you in German(Deutsch)
When they see you are sound in the language and also prove to them you can handle the job you applied for and make sure you can do this too not to just go and forge something sincerity will help you more than anything.

There are apps to learn this language like Duolingo and the rest but it is not as effective as when you are in a class and learn with other students also with a native speaker or one with good command of the language. Trying to speak to others in the class helps a lot that is why I will be introducing a school that can take you from A1 to B1 level.

For more info, you can contact me on 07020490767

Expand your coast