[Salvador Villalon] Add i18n lowercase locale

[salvillalon45]

Overview

This change adds lowercase support to change the passed locale code to lowercase.

It also adds changes where it changes en_us to en_US

Testing Done

I applied this code to double-check everything gets lowercase

window.piskel_locale = 'en_US';
console.log(window.piskel_locale);
console.log(window.piskel_locale.toLowerCase());

The browser console show the first string uppercase and the second string lowercase

[daynew]

Looks good, just one change needed! Merge after you make the change.