Translation:He is slimming himself down
This phrase "anajikondesha" literally means he/she slims himself/herself. Native Swahili speakers don't usually say he/she is on a diet in Swahili. It's a cultural context thing. "kukonda" (verb meaning to slim oneself). Keep in mind that Swahili is mixture of more than 1 language; some Bantu languages, Arabic words and a few English words. Swahili also takes into account the context of the situation one is referring to and the culture of the area. I am a native Swahili speaker from the Kenyan Coast. Hope this information helps.
I agree with you. The problem is that in Swahili they distinguish more clearly between "becoming thinner" and "causing to get thinner". Anajikondesha would mean literally "He causes himself to loose weight". I don't know any shorter way of saying this, and at the same time distinguish it from loosing weight because of, for instance, a disease.
Well, for most cases this would be correct, but it doesn't cover all possibilities. There are other reasons than dieting. He could be on a hunger strike. Then losing weight is self caused, but I don't think you would say dieting in that case.
I might be wrong, since I'm not a native speaker in English nor Swahili. But I don't think I am wrong.
You're right - of course you wouldn't describe hunger strike as dieting. Swahili is elegant enough to encapsulate both benign and destructive forms of 'thinning oneself' (which isn't a phrase in English) in one word. English phrases are nuanced according to the intent behind the slimming project! By 'he's on a diet I'm just suggesting the one used in perhaps 99.9% of cases. We also very commonly say "he's trying to lose weight" but that seemed a confusing suggestion when -jaribu is not part of the Swahili construction.
He is slimming would be correct (British) English according to the Oxford Dictionary of English:
verb (slims, slimming, slimmed) [no object] (British) make oneself thinner by dieting and sometimes exercising: if he's overweight, he should slim | (as noun slimming) : an aid to slimming.
And this should not be translated as 'She/He is on a diet" since there are other ways to loose (and keep) weight. Like exercising or doing hard physical labour.