Phrasebook entries can be very useful but there is a fear that useless entries defended as phrasebook ones will flood the mainspace. By requiring English phrasebook entries to be in other phrasebooks, we put limit on phrasebook entries, doing so with the help of an easy-to-verify criterion.
The lemming idea (being in other phrasebooks) is not applied to non-English entries since these may have too hard a time being found in phrasebooks, as pointed out by the opposition in Wiktionary:Votes/pl-2010-10/Phrasebook CFI.
Why "it can only be included if consensus grants an exception": This is an express override option allowing inclusion of English terms that are not in phrasebooks. Absent lemmings, the hurdle to overcome for inclusion is consensus (usually 2/3 majority), reflecting that phrasebook is controversial per Wiktionary:Votes/pl-2012-12/Removing phrasebook.
--Dan Polansky (talk) 09:18, 27 December 2018 (UTC)
Just a minor point: I think "sum" would work better. "Summation" sounds more mathematical and suggests an addition process rather than the result of adding things together. Equinox ◑ 13:00, 5 January 2019 (UTC)