Qt/FSUI: Add language flag icons to settings and setup

[SternXD]

Description of Changes

Added country flag icons to the language selection dropdown in the Qt UI settings. Each language option now displays its corresponding flag icon to the left of the language name.

EDIT: Updated all the GameDB flags to these new flags too

(SVGs came from this repository https://github.com/lipis/flag-icons/tree/main/flags/4x3 which has an MIT license)

Rationale behind Changes

Flag icons make it easier to identify languages in the dropdown, especially for users familiar with flags. The "System Language [Default]" option now shows the flag corresponding to the user's actual system language.

Suggested Testing Steps

1. Open Settings -> Interface and check the Language dropdown each language should show its flag icon
2. Check that "System Language [Default]" displays the correct flag for your system locale
3. Test the setup wizard language selection page flags should appear there as well
4. Make sure that special cases: "Español (Hispanoamérica)" shows Mexico flag, Serbian shows Serbia flag

Did you use AI to help find, test, or implement this issue or feature?

No.

[TheLastRar]

Some of the replacement flags for the regions aren’t correct.
I've pointed out a two that I've noticed immediately, but I've not checked against my full library of games.

[TheTechnician27]

I think what happened here is that PCSX2 used i18n codes, saw no code existed for Latin American Spanish, and took es-419 from IETF BCP 47. sr-SP is a perfectly valid i18n code, but it seems like it'd be easier if we just brought everything in line with IETF BCP 47 so we don't need a special case for Serbia.

[SternXD]

so looks like we can't change the country code because crowdin doesn't support IETF BCP 47 so we'll have to keep the special cases :(

[SternXD]

(Will push later today (I didn't end up pushing later that day 😭) still need to figure out what flag to use for the Scandinavian countries i could just use the old flag which isn’t that big of a deal for me but in that regard just wanted to see what everyone wanted)

[kamfretoz]

Works great.