> BTW, why after
Because the file simply contains a big \hyphenation {} pattern,
nothing else -- it doesn't explicitly switch to German.
> - and what about babel, which activates lots of stuff only at
> \begin{document}?
You have to load the file where German is active, perhaps with
\begin{otherlanguage}{german}
\input{dehyphtex}
\end{otherlanguage}
Werner