Default language of calendar subscriptions is based on your settings in Week Calendar (Settings > General > Language).
If the calendar isn't available in your language it will be in English automatically.
If you want your subscription in another language, please remove the current subscription:
1. Go to Settings on your iOS device
2. Go to Calendar (or Mail, Contacts, Calendars in iOS9 and below)
3. In the Accounts section go to Subscribed Calendars
4. Select the calendar you wish to remove
5. Tap on the red Delete Account button
and then add it again, it then will be based on your language settings automatically.