By default the game uses either Mac OS system language or just International localization (can't check - they both are the same on my Mac). But there is no way to change it:
- no option in game menu;
- manual editing of "Mac-HeistEngine.ini" does nothing (but the file is not rewritten after launch);
- attempt to bind App's language via System Preferences (Language & Region / Apps) tells the Game doesn't support additional languages.

I see l10n resources in app bundle. Moreover there are CookedMac folder where I can see something that looks like baked scripts. But no way to hook preferred language there. Renaming all localization files may help (placing them as new INT) - but I didn't try yet.

Have anybody found proper solution?