It's much harder to to implement a language in a proper way than writing a widget library.
But that hardness comes from the level of knowledge one must have in different fields such as compiler theory, cathegory theory, hardware, beside classical computer science and coding.
To write a widget library you just have to know how to code and have some experience.
That being said, the amount of work for implementing a widget library might be higher, but is less qualified work being done.
I dabbled in computer language theory and compilers and it's hard. I have no issues reading source code for widget and GUI libraries.
But that hardness comes from the level of knowledge one must have in different fields such as compiler theory, cathegory theory, hardware, beside classical computer science and coding.
To write a widget library you just have to know how to code and have some experience.
That being said, the amount of work for implementing a widget library might be higher, but is less qualified work being done.
I dabbled in computer language theory and compilers and it's hard. I have no issues reading source code for widget and GUI libraries.