I have written an lthenc.dfu file to be able to use thai along with
\usepackage[utf8]{inputenc} (I attach it and hope that is doesn't
get lost). What is the best way to add it to the system? (I don't
care about my name in the file. If if is simply added to the base
distribution like other dfu-files that's fine for me.)

Why trying out the dfu I saw that thai.ldf changes the catcodes of
all non-ASCII-inputs and so breaks the catcode changes of inputenc.
So the .dfu only works if inputenc is loaded after babel:

%\usepackage[utf8]{inputenc} %doesn't work

(I wrote the maintainer about the catcodes some days ago but this
get an answer yet).

Ulrike Fischer