I've installed Windows 10 official release (not the preview) from Windows 7 Ultimate SP1 and I've some problems with the language. My OS is in French but all my modern-ui apps (weather, store, calendar,...) are in English. I've W10 Professional and I precise that when I had W7, I had to set default language to French after the installation (this was an english version of Windows). Have you some ideas ?
It's a late answer, but when I had the same problem I solved it very quickly and easily. I live in Germany and love my OS to be in English. In order to achieve a full English OS, including apps, and a German keyboard layout I went into my locale settings (Settings => Time & Language => Region & Language) and first off moved English (which was set to override the OS language) to the top. Then I clicked on English, clicked on Options and directly clicked on Add a keyboard and selected the German keyboard layout. After that was set I removed the US keyboard layout and then had Windows 10 displaying in English, including all of the apps, with a German keyboard layout.
I hope this answer alone might help or would give you some options to look after.
My problem was that I did not set my default language as the one that I wanted Windows 10 apps to be in.
For Windows 10 users:
Settings > Time & language > Region & language > Click the language that you want it to be default > Set as default
Then after you restart the Windows 10 app, it should use the default language setting.