keyboard keeps resetting itself to U.S
The default key command to switch Input Sources is  Cmd ⌘   space  which you may be triggering accidentally.
From comments, I understand the new default since El Capitan is  Ctrl ⌃   space , however it also appears it will remain at the old setting if you upgraded from an older OS.

To remove unwanted input sources go to System Preferences > Keyboard > Input Sources &amp; remove all except the one you wish to keep.



Alternatively, if you want to keep the other sources but not trigger them accidentally, then remove the shortcuts in System Prefs > Keyboard > Shortcuts > Input Sources. [This option is only visible if you have more than one source in the previous pref pane.]



With the key command to change sources removed, you can change from the menu bar flag.  

If you remove all except one source, the flag icon will disappear.