There are no builtin options in group policies to change system locale (language for non-unicode programs) on Windows 10 / Windows 7, so you have to use registry.
These three registry values control the system locale:
To find out necessary registry values, change system locale to needed on a test computer, watch these values change and then deploy them using group policy preferences. After these registry values are changed, restart is required to take affect.
For example English (United Kingdom) has following values: