Language supplementation is a process where we copy missing translations in a particular language X from existing texts in language E.It is recomended to carry out language supplementation after carrying out any of the below activity :-
a.Importing EHPs
b.Installing a new language.
c.Importing Support Packages.
d. any tranport activity which affect language.

SAP has give the customer choice to define which language they want to use for Supplementation. So may be a customer in Poland selects DE for supplementation of PL because his employees do understand DE too but do not understand EN.

The Language Supplementation has to be started manually from SMLT.