Unclear, which language code standard is used

[rostopira]

Please, add to readme some ISO link or whatever, because it's unclear which standard is used
I were going to add Ukrainian translation, but it uses uk and ua codes in different standards

[andresaraujo]

This is a good point I did not put much attention when I first created the library. I was following https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes for my own reference, and I think all current locales adhere to that