The various contributors have described the chronology of the work's
development well.

To clarify any outstanding ambiguity in answering the original question:
as Mendelssohn worked on his musical revision as the English translation
progressed; and as the first performances of the revised version we hear
today were performed in English, both languages are equally acceptable,
and "Elijah" is just as defensible as the German equivalent - not that
any defence is needed for the perfectly sensible proceeding of singing
in the vernacular.

What a pity opera composers didn't take the trouble to work bilingually, as
Mendelssohn did here.  How many hot air balloons on "opera in translation",
surely the dullest of all musical chestnuts, might have been avoided.

Then again, thinking of Gluck, Verdi, Wagner and the musicological chaos
caused by their "Paris versions", perhaps its just as well that most
composers have stuck to their native tongue ...

Christopher Webber,  Blackheath, London,  UK.
http://www.nashwan.demon.co.uk/zarzuela.htm
"ZARZUELA!"