Something like this should fix everything in one go:

find . -type f -exec sed -i \{\} -e "s/an other/another/g" \;

Well not really...

Only the 'source files' have to be changed (ChangeLog must not be touched, for
instance). And 'than others' must also not be changed int 'thanothers', right ?

But, it is not clear for me if "an other one" should not be kept when we want to
mean "a different one", "another one" being more equivalent to "an additional one".

Any English native speaker around ?

Done in commit 147cc20