In the tips and notes it sometimes says phrases don't translate well and should be treated idiomatically (e.g. "beth sy'n bod"). I think it would be useful to give the idiomatic and literal translations with these phrases as they may tive an insight into how some Welsh sentences are formed, what the individual words are, and just for the sake of curiosity.
I also agree that knowing the literal translation is helpful, knowing how the target language expresses things. Otherwise you try to translate literally things that are thought of in a different way. Do you catch a cold? or does the cold catch you?
If the system prevents explanations being put in the Tips and Notes, there is a way round - ask in "Discuss sentence". I've seen quite a few queries about idioms in the Welsh tree and the volunteers are extremely good at clarifying them. Thank you, volunteers, for your hard work!
I very much agree that the literal would be useful as well. I feel like I am memorizing too many chunks of language without knowing how the pieces fit together to make other sentences.
I think it would be useful to give the idiomatic and literal translations
Unfortunately, volunteers taking care of the course can't do that.
It'd need for Duo staff to change/improve the system. Therefore, I advise to move this suggestion into the general “Duolingo (in English)” forum.