How do I decrypt a DLL?

The outer .dll would then decrypt the encrypted .dll with the key - if signature verification passes. Load and use the .dll. There is nothing wrong with this method, per se. However, as others have pointed out, it is certainly possible just to wait until the inner DLL is decrypted and then dump it from memory.

What is a DLC?

DLC ，是Downloadable Content的缩写，意思是游戏追加内容，可以认为是游戏的一些额外剧情。 可以通过eshop购买，和数字版的购买方式是一样的。 比如国行可以玩塞尔达外服卡带，但因为不支持DLC，你就不能骑摩托，少了一点点游戏体验。

What is decryptdecryption?

Decryption is the reverse of encryption; it is the transformation of encrypted data back into some intelligible form. Encryption and decryption require the use of some secret information, usually referred to as a key.

Can I use DLC files offline?

DLC files (by design) require a webservice to be decrypted. This means you can't use them offline. This is not a limitation of this application but an intended feature of DLC files. Written in the page you linked. The Github repository is just interacting with this website, presumably made by who wrote the program. We know, we know.