[Enhancement] Support book language being case insensitive

[ZLoth]

Describe the issue

The source of the issue is from the metadata import where sometimes the language is entered in as "english" instead of the proper "English". Per https://www.sussex.ac.uk/informatics/punctuation/capsandabbr/caps#:~:text=(c)%20The%20names%20of%20languages,%2C%20French%2C%20Italian%20and%20Portuguese. , the name of the language is capitalized. However, Audiobookshelf treats "English" as a seperate language from "english".

Steps to reproduce the issue

Recording of the issue: https://github.com/advplyr/audiobookshelf/assets/6700159/446d7979-34e8-4130-a43e-cd979ce1fa51

Example book:

Audiobookshelf version

2.9.0

How are you running audiobookshelf?

Docker