"weka" doesn't mean add. "kuongeza" means to add. Sometimes whoever made this isnt totally correct. but then again there are variations of swahili used. Congo and kenya use swahili similarly while tanzania has a much more formal and strict way of speaking swahli. i lived in kenya for five years.
You should have opened with that. Much clearer. :)
When the team started there were two native Swahili speakers and one native speaker of English (not including the one that dropped out early on). These weren't just native speakers, either, but language teachers. Naturally I assumed the Swahili would be correct if nothing else.
I don't think there have been many Tanzanian speakers using the course. Most of the Swahili speakers I've seen comment speak Kenyan, and on further investigation often what they claim is wrong is just dialect. But it does seem that there's a fair amount of bad Swahili too, and not all of it can be blamed on the course having been rushed out at the end.
So, "put salt" does not make sense on its own, and "add salt" is incorrect as a direct translation. But does "add salt" convey the meaning of the sentence fragment, or is it completely wrong on all levels?
I learned Swahili in Tanzania, I was also taught that Kenyans did not know how to speak Swahili and to make sure I didn't pick up on any of their poor speaking skills. LOL
That said, the biggest problem I see with learners on Duolingo in general, especially here on the Swahili course, is that they get cemented into the idea that Swahili word 'x' = English word 'y.' I've repeated it over and over, but this is a TERRIBLE way to learn languages. Kuweka does not mean "to put," because kuweka was not structured around the English language, otherwise Swahili speakers would be speaking English. Instead Kuweka conveys a thought, just like how 'put' in English conveys a thought. The thought that Kuweka portrays would translate to an English thought that is similar to 'put.' But when we string multiple thoughts together, the words we use often changes. For example right here, you can say you are literally putting salt (on food). So it's not that the translation is wrong, in fact the translation here is better than 'put salt,' because it translates a recognizable Swahili thought into and English thought that is also recognizable. That is a very good and well sought after translation skill.
I have a book here "Mapishi ya Kisasa" written in Swahili (not translated), by Tereza K. Zani (from Mombasa at the time the book was written). She uses "-tia chumvi" and "-ongeza chumvi." I don't see any "-weka chumvi." She does use "-weka" as in "...weka vyakula kwa siku za mbeleni" and "-weka maembe katika madebe". Of course, this is just glancing through the book. I have not done a line-by-line analysis. "-tia chumvi" is clear in the context of a book of recipes, although it does have an idiomatic meaning too. Inasemekana ripoti hizo zimetiwa chumvi -- It is said that these reports are exaggerated.