In the Purchase Order, you can see the language in PO Header - Communications data - Language.

If the Purchase Order have been saved before you change the Vendor Master Language, the PO need to be closed as the language in the PO Header cannot be changed.

This is the SAP Language Error that will appear when creating PO with language mis-matched:

No short text maintained in language CS (please re-maintain material ABCDEFGHIJK)
Message no. ME 095

Diagnosis
No material short text (short description) has been maintained in language CS.

System Response
No short text can be adopted from the material master record.

Procedure
If a material is to be procured from a vendor in a different language, the material short text must first be maintained in this language.

Maintain the short texts in the material master record in those languages in which procurement is normally carried out. In this case, you should at least maintain the short text in language CS. 

See also
Item Overview Of A Purchase Order

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