Exegesis:

tines de tōn Pharisaiōn eipan ‘some of the Pharisees said.’ Their presence in the grainfields is not explained; it is simply assumed that they are there.

ti poieite ho ouk exestin tois sabbasin ‘why do you what is not allowed on the sabbath?.’ The question is directed to the disciples.

exestin impersonal verb ‘it is permitted,’ in Luke always of what is permitted, or, with negation, what is forbidden by the law.

Translation:

But some of the Pharisees said. Since the Pharisees are new to what is narrated in this section, it may be necessary to introduce them as such, e.g. ‘then there-were-present some Pharisees who said’ (Malay), cf. also on 1.5.

Why are you (plur.) doing what is not lawful to do does not ask for information, hence, ‘Why! (or another exclamation expressing indignation or disapproval) You are doing…’ may be preferable. For similar reasons Toraja-Sa’dan restricts the question to the first clause, ‘What are you doing?—It is not to be done.’ For what is not lawful to do, or, with a further shift, ‘what we (inclus.) should not do.’ In countries with a Muslim background a technical term used in connexion with Muslim law may be an appropriate rendering, e.g. ‘what is not halal (i.e. permitted)’ (Bahasa Indonesia), or, ‘what is haram (i.e. forbidden).’

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.