We Persian speakers have a common idiom, Khaste Nabaashid, and usually say it to someone who finished a task or is in the middle of doing that. The literal translation of the idiom is something like don't be tired or I hope you do not feel tired.
Most of time non-Persian speakers who learnt Farsi become confused when we say them Khaste Nabaashid. So, I think there is no equivalent for that in other languages, specifically English.
Is there any idiom that conveys this?
Update: I just read somewhere, the equivalents may be nice job or well done! However, I'm not OK with them.
Update 2: As a given scenario, it could be used as greeting! When we see a friend who's doing some work, we usually say, Salaam (Hello), Khaste Nabaashid!
Moreover, if she/he is doing something for us, the idiom conveys our appreciation, otherwise it's used as some type of admiration.
Update 3: This expression implicitly signals the end of the work. For example, when students are tired and want to indirectly and politely ask the teacher to stop and end the course, they usually use this expression and tell him/her Khaste Nabashid!