Proton seems to load Latin American localisations of games despite Steam being set to Spanish - Spain (Dark Souls 3, probably more)

Compatibility Report

• Name of the game with compatibility issues: Dark Souls 3, probably more though
• Steam AppID of the game: 374320

System Information

• GPU: GTX 1070
• Driver/LLVM version: Nvidia 410.73
• Kernel version: 4.18
• Link to full system information report as Gist: https://gist.github.com/Leon422/15f1906b04e48ab3d03e6b946e008979
• Proton version: 3.16-4 beta AND 3.7-8 (both tested)

I confirm:

• that I haven't found an existing compatibility report for this game.
• that I have checked whether there are updates for my system available.

Symptoms

Despite Steam being set to Spanish - Spain and the game being set to Spanish - Spain in its properties, the game is loading the Spanish - Latin America translation. This doesn't happen on Windows.

Reproduction

Load Dark Souls 3 in Spanish - Spain under Proton. You can tell it's the wrong language setting right at the "Press any button" screen. It should be "Pulsa cualquier botón", as that's the Spain translation and that's what is loaded on Windows. However you get "Presiona cualquier botón", which is the Latin American version of the text.

Steam recently split the Spanish language in two so maybe Proton doesn't know how to handle it?

Reproduce Dark Souls 3 (AppID 374320) with Spanish - Spain selected under Proton 3.16-4 beta or 3.7-8, and compare the opening text with the Windows result. Start by tracing how Proton handles Steam's language selection; done means the Spain translation is loaded, including “Pulsa cualquier botón,” rather than the Latin American wording.