I've been thinking but I haven't come up with any way the Eliot's facility
with foreign languages actually helped his writing poetry in English.
As for his use of the foreign phrases though he often combined several
languages rather close together. Mostly epigraphs but then there is the
closing of TWL. The quickest switch must be in the later dedication to
Verdenal where he has English and French in the first line and then goes on
to a quotation from Dante in the Italian.
It seems to me to the by using the languages so close together E. was
striving to show a universality in life.