I completely agree. I suggest to amend the Coding Style documentation page to clarify this and add an example under "Language strings > Structure": https://docs.moodle.org/dev/Coding_style#Language_strings.
Is there some procedure to follow (e.g. "policy" issue to open, as this is a quite official document) or can I do it directly?
Yes, there is a procedure - please report such a suggestion as a new MDL issue, component "Policy".
I agree it is a bad practice to design strings with trailing and heading whitespace and I believe it should be considered a bug in the original English strings.
Meanwhile, you should be able to insert strings with whitespaces into AMOS though - https://lang.moodle.org/mod/forum/discuss.php?d=5908#p6932
Thanks David for (pointing to) the tip : Maintaining CTRL before clicking on text editing place to avoid AMOS filtering
Here's a command to search for english files containing trailing space (launch from Moodle dir) :
for i in `find . -type f -name '*.php' | grep 'lang/en/'`
do grep -Hne " ';$" $i | grep string
done | sort | cut -c 3-
There are more than for begining space. I suppose my search won't find multiple lines strings...