Items are not ingested into default folders (Inbox, Sent Items, etc) in user mailbox when there is a language mismatch.
If the items were archived in Enterprise Vault using the Native language for the mailbox then names of all the folders like Inbox, Sent Items, etc. are also stored in the Enterprise Vault Directory database in Native language.
When these items are then migrated to a User mailbox which is using a new default language, the user will have two Inbox folders: one in the new default language and the second one in the Native language. This is presently by design. Archive Shuttle extracts the data from the source archive and gets the folder names from there, and ingests them into the target container with those same folder names.