This issue occurs with the hidden "Platform Integration User" which is assigned to the Data 360 Salesforce Connector permission set upon creating a CRM connection with Data 360. This Permission set controls access to the CRM objects and fields that ingest into DC. Basically a user can change the language settings for this Platform Integration user which isn't advised as it can lead to unknown behaviour with other components that depend on that user for integrations. This explains why you can see a different language when creating fields for a CRM data stream as this user as this is the integration user between the two clouds.
Unfortunately at this time, we do not allow field labels to be changed after creation, but the engineering team hope to address this in the near future.
The user will have to recreate their data streams where during data stream creation time, you can adjust your labels. Labels are not system critical in this case.
We would also recommend resetting that user back to the default language for the purpose of integrations.
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