Add Serbian latin (sr@latin) translation.

Created by: Faenriis

Hi, bit of an explanation why I'm sending a pull request instead of using Weblate.

Serbian translation has two variants, for cyrillic and latin script. As defined in glibc (on top of which Gnome and Gradio is built, and uses its locale definitions) defines them as sr (or sr_RS) and sr@latin (or sr_RS@latin). You can check locale definitions that are provided as a part of glibc (usually in /usr/share/i18n/locales/ ).

However, Weblate offers only sr, sr_Cyrl and sr_Latn. No problems with sr, I translated it there, and I used provided form to request sr@latin (also provided explanation why - twice), but few weeks later, I still got no response from Weblate admins. Therefor, I'm resorting to sending this pull request. If you have any suggestions on how to resolve the problem by other means I'm all ears, but if you don't, please consider merging this commit at least as temporary solution until proper fix is found. Thank you.