Exegesis:
ho de theos ou mē poiēsē tēn ekdikēsin tōn eklektōn autou ‘and will not God protect his elect?’ de is contrastive. ou mē with subjunctive in an interrogative clause expects an answer in the affirmative, poiein tēn ekdikēsin with following genitive is equivalent to ekdikein with accusative (cf. on v. 3).
eklektos (also 23.35) ‘chosen,’ here in the plural referring to the people of God as his chosen community.
tōn boōntōn autō hēmeras kai nuktos ‘who cry to him day and night,’ going with tōn eklektōn. The clause refers to a situation of persecution and affliction. boaō with dative means here ‘to cry out in prayer.’ hēmeras kai nuktos, temporal genitives, means here virtually ‘continually,’ ‘uninterruptedly.’
kai makrothumei ep’ autois ‘and will he delay over them.’ This difficult clause (see commentaries, esp. Plummer) is best understood as part of the interrogative clause, the expected answer being now in the negative. makrothumeō is best understood as meaning ‘to be long in helping,’ ‘to delay’; autois refers to the chosen.
Translation:
The answer to the two interrogative clauses is to be found in v. 8, to the first an explicit, positive answer, i.e. “he will vindicate them”, to the second one that implies a negation, i.e. “speedily”, i.e. not delaying long. The form to be given to the clauses in translation must be in accordance with the subsequent answer. Cf. for the first clause e.g. ‘how about God? won’t he certainly avenge (cf. on v. 3) as many as have been chosen by Him…’ (Tzeltal), ‘God will-he-not-reveal-as-true his chosen ones, won’t he?’ (Fulah, anticipating ‘no, he will reveal them as true’ in reply).
His elect, or, ‘his chosen ones,’ ‘the ones he has chosen,’ ‘those he has pulled from amongst others’ (Sranan Tongo). The aim of the choice may have to be specified, e.g. .’.. chosen to belong to him, or, to be his people/children.’ Cf. also on “chose” in 6.13.
Cry to him, or, “appeal to him” (Phillips), ‘cry, or, call to him for help’ (cf. Good News Translation), ‘pray to him fervently (for help).’
Day and night, see on 2.37.
Will he delay long over them, or, ‘and will He-cause-to-be-long the waiting of those people’ (Tae’), ‘will he delay in responding to them’ (Tagalog), ‘he will-be-long in-helping them, will he’ (Fulah, anticipating ‘no, he will not be long…’ in reply), ‘won’t He immediately answer their words’ (Tzeltal, this positive wording being felt to be more forceful).
Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.