Windows 10 search results appear in wrong language

I just ran into this myself. Try rebuilding the 'index', this is where the start-menu searchresults are stored.

http://www.winbeta.org/news/how-rebuilding-your-search-index-windows-10

I had a similar problem (actually, my entire laptop was in Thai). I did the following steps:

To open Settings, go to the Action Center (the icon in the taskbar beside the clock) and press All Settings.

1. Go to Settings → Time and Region → Region and Language → Delete all other languages and only keep English. My keyboard layout was also Thai, so that can be changed here too.

2. Control Panel → Region → Format → Set it to English

3. Control Panel → Region → Location → Set it properly

4. Control Panel → Region → Administrative → Change system locale → Select English

This was the most important step. You may reboot after this.

5. Control Panel → Region → Administrative → Copy settings → Check both boxes and click OK

Again reboot! After doing all this German should vanish entirely from your laptop. After this, make sure you have deleted German language and selected override English as language everywhere just to be sure.

When you change the language in Windows not all the nooks and crannies of the OS are affected. I haven't run into your problem before, but I would try copying the language setting to other parts of Windows. Here's how.

1. In the Cortana box type Control Panel.
2. Select the heading Clock, Language, Region.
3. Under the Region heading select Change location.
4. A Wizard dialog will open, click on the Administrative tab and press the Copy Settings button.
5. On the wizard page fill the two checkboxes at the bottom of the page and click OK.