[docstrings] fix project-wide English language mistake
Even though 'u' is a wovel, it may be pronounced 'yu' which is a consonant
sound. In such cases precede it with 'a' instead of 'an'.
[test/i18n] Update reference po file following 75cd7df5c32e
This changeset added a (missing) message "add a <etype>", so reference po file
needed an update.