Last updated July 2019.
You may see two contact folders in People, one in English and the other in your localized language. One of these folders may not sync to your devices.
Note: Contacts created in People can be synced across other devices and apps. Imported contacts (for example, contacts from a connected account) aren't synced.
Follow these steps to ensure that all your contacts sync:
Make sure your Outlook.com language is set to English. To change it, go to Language and time settings.
Export your contacts from the folder which isn't syncing.
Import your contacts again. The contacts will be automatically imported to the correct folder which is syncing.
Change your Outlook.com language back to your personal preference.