There is limited number of supported locales.

Google Chrome/Chromium/Vivaldi supports:
ar, am, bg, bn, ca, cs, da, de, el, en, en_GB, en_US, es, es_419, et, fa, fi, fil, fr, gu, he, hi, hr, hu, id, it, ja,
kn, ko, lt, lv, ml, mr, ms, nl, no, pl, pt_BR, pt_PT, ro, ru, sk, sl, sr, sv, sw, ta, te, th, tr, uk, vi, zh_CN, zh_TW

Opera supports:
af, az, be, bg, bn, ca, cs, da, de, el, en_GB, en_US, es_419, es, fi, fil, fr_CA, fr, fy, gd, he, hi, hr, hu, id, it,
ja, kk, ko, lt, lv, me, mk, ms, nb, nl, nn, pa, pl, pt_BR, pt_PT, ro, ru, sk, sr, sv, sw, ta, te, th, tr, uk, uz, vi,
zh_CN, zh_TW, zu

Firefox supports:
ach, af, an, ar, as, ast, az, be, bg, bn_BD, bn_IN, br, bs, ca, cs, cy, da, de, el, en, en_GB, en_US, en_ZA, eo, es_AR,
es_CL, es_ES, es_MX, et, eu, fa, fi, fr, fy_NL, ga_IE, gd, gl, gu_IN, he, hi_IN, hr, hsb, hu, hy_AM, id, is, it, ja, kk,
km, kn, ko, lt, lv, mai, mk, ml, mr, ms, nb_NO, nl, nn_NO, or, pa_IN, pl, pt_BR, pt_PT, rm, ro, ru, si, sk, sl, son, sq,
sr, sv_SE, ta, te, th, tr, uk, uz, vi, xh, zh_CN, zh_TW

If you use an unsupported locale, browser (at least Chromium-based) will ignore it.

Up to date Chrome supported locales list:
https://developer.chrome.com/webstore/i18n?csw=1#localeTable
