There is actually no magic in it. Before a mailbox import request language settings should be the same on the target mailbox ( where you import to ) and the .pst file you are going to import. This prevents double default folders being displayed in 2 languages ( one folder for each language )
Simply set the preferred language on the target mailbox. The following command will set the language, date format, time format and time zone to german language.
 [ For OnPremise leave the -Organization or -OrganizationalUnit switch away. More on syntax: MutliTenant VS. OnPremise ]
The default parameters for english / US are:
-language : en-US
 -dateformat : M/d/yyyy or MM/dd/yyyy
 -timeformat : h:mm tt or HH:mm
 -timezone : “Pacific Standard Time”
The LocalizeDefaultFolderName switch turns the default folders to the set language. If you set it to $false the default folders will not switch to the new language.
The mailbox import request language settings shouldn’t be a problem now. Trigger the import of the .pst using the following CMDlet :
New-MailboxImportRequest -Name GiveItAName -BatchName GiveItAName -FilePath \\ServerName\Sharename\FolderName\PSTNameToImport.pst -Mailbox TargetMailboxName
If the import request fails you can play with the following parameters :
-BadItemLimit : Specifies the amount of corrupted items to be ignored without failing the import request.
 -AcceptLargeDataLoss : Needed if you set the BadItemLimit above 50.
 -LargeItemLimit : Specifies the amount of large items to be ignored without failing the import request.
 [ LargeItemLimit is not available in Exchange 2010]

