Before adding a new sentence, please search by keyword to confirm whether it brings new vocabulary to this site.
Literal
New sentence [object-を] add before [at-に], that sentence [by-によって] this-site [to-に] new vocabulary [subject-が] brought whether [question-かどうか], keyword search-[and-して] please-confirm.
A site instruction asking contributors to vet new submissions. もたらされる is the passive of もたらす ('to bring about, deliver'), here describing whether the sentence 'brings' new vocabulary to the site. ~によって marks the means or agent — 'by means of that sentence.' かどうか ('whether or not') frames an indirect yes/no question. ご確認ください stacks the honorific ご onto 確認 ('confirmation') with the polite request ください — standard formal-instruction register.