"Os gweli di'n dda"
But "exactly what they are saying" is "please".
If you say "Goodbye", you are not literally saying "God be with ye", are you? You're just bidding someone farewell.
I doubt that Welsh people think about the literal translation of os gweli di'n dda any more than English people do about "goodbye" or "o'clock".
"Os gwelwch yn dda" is more formal and infact could be said "Os gwelwch chi'n dda" but the chi is often omitted becuase the -wch ending tells us that it is formal without using "Chi". "Os gweli di'n dda" is the informal and uses "di" instead of "chi". I don't know why we don't omit the "di" like we do in the formal version but it most certainly would never be done.
To give a fuller explanation: Using os gwelwch yn dda / os gwelwch chi'n dda / os gweli di'n dda is more formal than using plis. And then os gwelwch yn dda / os gwelwch chi'n dda would be use with one personal formally/respectfully or to more than one person with no reference to formality or respect. Os gweli di'n dda would be used with an individual we are more familiar with or have no need to show respect too.
Duolingo accepts "if you please" as a translation for "os gwelwch chi'n dda". However, it doesn't for "os gweli di'n dda."
I was taught to mentally treat "os gwelwch chi'n dda" as "if you please". I don't remember why, exactly. Regardless, for learning I found it useful. Others might stumble across this as well, so I thought I'd mention it. (I'm not sure how concerned about this I am myself since I expect that, in the wild, I will either use or encounter "os gweli di'n dda" precisely never.)