Following this link will force OmniOutliner to use English. The app will need to be fully quit and relaunched for this to take effect. To quit the app, return to the home screen, double-tap the home button to bring up the multi-tasking bar, tap and hold on the OmniOutliner icon until the "-" image appears and tap that to quit the app. Then relaunch OmniOutliner like usual.
Reset language for OmniOutliner
Following this link will reset the preferred language so it uses the system default. The app will need to be fully quit and relaunched for this to take effect. To quit the app, return to the home screen, double-tap the home button to bring up the multi-tasking bar, tap and hold on the OmniOutliner icon until the "-" image appears and tap that to quit the app. Then relaunch OmniOutliner like usual.