How do I setup custom locale?

Hello!

I’d like to forward you to my recent comment on this topic; do replace every instance of “en_NZ” with “ru_lv” in your mind:
https://wordpress.org/support/topic/oglocale-problem/#post-11456346

There, you’ll also find the filter to set your custom locale. Although, I doubt you’ll want to implement it after reading my comments.