Steps to change the language & regional settings on Office 365 Server
One of the reasons to change the language
For Instance: Your source Inbox folder is named 'Boîte de réception', which is in French language. If you migrate this folder directly to the destination server then by default it will not get mapped to the 'Inbox' folder on target server. In order to map this to the respective folder, change the regional settings on Office 365 target server to that of your source mailbox folder language (in this case, it is French).
Steps to change the language settings:
1. Login in to your Office 365 account.
2. Click the Outlook button under Apps

2. Click on the Settings menu on the top right of your screen

3. In Settings, search for "Language and Region"

4. From the Language drop-down menu, select the language.

5. Check the option Rename default folders so their names match the specified language.

6. Click the "Save" button.
7. Your default system folders should now be renamed to the selected regional language.