Q: Is it important in which client a language import or supplementation is performed?
A: The language import must always be performed in client 000. The supplementation must first be performed in client 000 and can then be scheduled in higher clients. However, only client-specific tables are supplemented.
The language guide points out that language supplementation in clients in production should be carried out before the customizing only. This way, the texts of the SAP reference customizing are supplemented but not the texts that have been added at the customer's in the frame of customizing since those texts still need to be translated. In the translation environment (SE63), the translation process runs more smoothly when the texts have not been supplemented.

If customer customizing texts are not to be / cannot be translated (e.g. time problems in translation before the going-live, no translator available), it is advisable to carry out language supplementation in those clients. This way, failures in various transactions (blank input help screens, transaction terminations) can be prevented.