(oops, sorry i should have put it here. ) In English, 'simple' itself has not only one meaning, hence the Korean has more than one translations accordingly. 1. simple, which is not difficult or complicated, but easy to understand and simple. 문제가 간단하다. simple problem(which you can understand very easily.) 2. simple, plain(personality) not artificial or luxurious(appearance). 그사람은 소박하다. can imply two things, either 'that person is simple-minded, innocent(personality)'; or 'that person is not keen to look better with artificial stuff in terms of appearance'. Maybe if this used on a girl, it means she doesnt put makeup, but keeps herself very natural and plain. Really, it depends on the context. 간단/소박하게 먹자 or 간단/소박하게 살자 both are correct, there is no significant difference in terms of these two groups, alright? the nuance lies on the part which you rally want to emphasize. Hope this makes sense to you!

in English, 'simple' itself has not only one meaning, hence the Korean has more than one translations accordingly. 1. simple, which is not difficult or complicated, but easy to understand and simple. 문제가 간단하다. simple problem(which you can understand very easily.) 2. simple, plain(personality) not artificial or luxurious(appearance). 그사람은 소박하다. can imply two things, either 'that person is simple-minded, innocent(personality)'; or 'that person is not keen to look better with artificial stuff in terms of appearance'. Maybe if this used on a girl, it means she doesnt put makeup, but keeps herself very natural and plain. Really, it depends on the context. 간단/소박하게 먹자 or 간단/소박하게 살자 both are correct, there is no significant difference in terms of these two groups, alright? the nuance lies on the part which you rally want to emphasize. Hope this makes sense to you!

간단하다 means "simple; easy; uncomplicated; brief; quick; light" 소박하다 means "simple; artless; naive; pristine; idyllic; frugal" 소박하다 has the nuance of "honest, greedless" 간단한 식사 means "a light meal(regardless of cost)" 소박한 식사 means "a frugal meal(low cost)" 그 문제는 간단해. (The problem cannot be honest or greedless) The problem is simple. 그 사람은 참 소박해 He is so greedless and honest. cf. 그 사람은 참 단순해 He is really a simple-minded person. 저녁은 간단하게 먹자. Let's have a light meal. 우리 소박하게 살자. Let's live greedlessly and honestly.

'간단' = 'simple' and '소박' = 'austere', 'artless', 'plain' or 'rustic' :)