fix COMPILERS exts alias creation

splitext assumed the ext was a apth, not ext, so a small path
"f" had to be added first
fix recongnition of files with a longer ext

splitext would only get the shortest ext, so endswith() has to be used
add a test case for posts only in secondary languages.

this test however only checks that nikola runs and creates something
it cannot check that titles are translated (as we remove the
translation)
flake8 fixes

remove whitespace and do not redefine ext in list comp