Tl;dr `lang-` must be lowercase, capitalization on the language name can be anything

-----

Let's try it out [in the formatting sandbox](https://meta.stackexchange.com/a/381439/1081494):

> ### All lowercase:
> ~~~
> ```lang-python
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ~~~
> 
> ```lang-python
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ### All caps:
> ~~~
> ```LANG-PYTHON
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ~~~
> 
> ```LANG-PYTHON
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ### Language name all caps:
> ~~~
> ```lang-PYTHON
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ~~~
> 
> ```lang-PYTHON
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ### All mixed caps:
> ~~~
> ```LaNg-PyThOn
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ~~~
> 
> ```LaNg-PyThOn
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ### Language name mixed caps:
> ~~~
> ```lang-PYTHON
> for i in range(10):
>     if i % 2:
>         print(i)
> ```
> ~~~
> 
> ```lang-PyThOn
> for i in range(10):
>     if i % 2:
>         print(i)
> ```