Keigo
Keigo is a type of formal language that the Japanese use when speaking with older people and superiors. Mastering it is super-difficult, even for many Japanese natives, but there are classes that foreigners can attend to learn it. However, there are two problems most foreigners experience when learning Keigo.
First, it’s incredibly difficult to learn on top of understanding how to speak fluent Japanese. You have to learn how to use it, otherwise, people will see you as being rude. Second, you might end up using it for any situation, including speaking with friends. Though not rude by any means, it can be awkward.