I do not know where you tell the system what the default language would be, I dont think there is such setting, maybe you developed such a routine for your printed documents.
The material short text is taken based on the logon language
the PO item long text is taken from the last language found.
that is the based on programming, the read statement reads the text in all the languages in alphabetical order of language key, which is internally just 1 byte long e..g for ZH it is 1, E for EN, S for ES, and brings back the last read record if no dedicated record was found.
If you have no short text in the language of the vendor, then you usually get error message ME095 - "No short text maintained in language ....