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The root cause in this case is file joining enabled at the stage of creating the said XTM project in the XTM Cloud UI, regardless of the type of joining applied. This is an expected behavior that in such projects it is not possible to add or delete target languages.

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Good to know!

File joining settings can be enabled and managed in the XTM Cloud UI, in Configuration → Settings → Projects → Joining files in sets (administrative privileges required).

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