Weekly build 65604 (which is what I was using) does have a .mo
file for English and it is identical to the one in the repository:

marten 805% ls -l ~eiffel/Eiffel6.0.65604/studio/lang/mo_files/
total 216
-rw-r--r--   1 eiffel        315 Dec 14 00:30 en.mo
-rw-r--r--   1 eiffel      42305 Dec 14 03:28 fr.mo
-rw-r--r--   1 eiffel     152939 Dec 12 19:23 zh_CN.mo

English works fine when I start estudio with general.local preference
indicating "en".  But when I exit estudio, the preference is overwritten
with "zh_CN".