Can a person(a politician specifically) be described as diplomatic as well as forthright at the same time? Or do the two attributes contrast with each other?
In general the two attributes contrast one another. In other words a diplomatic statement would be a "flowery" forthright statement. But of course for a particular statement it depends. A particular statement could be diplomatic as well as forthright.
They have very different meanings. In some ways they are opposites.
Diplomatic (when it doesn't mean "of diplomacy") means carefully phrased to avoid offence.
When we inform the parents that their children will move to a low maths group, we will need to be very diplomatic. Make sure we use langauge like "this group will ensure your child receives the proper support".
"Forthright" means direct and strongly expressed.
He made a forthright defence of the Brexit policy.
Being diplomatic suggests being subtle and indirect. The opposite of "forthright".
If something was forthright and diplomatic, it would suggest "strongly expressed" but "carefully phrased so as not to offend". That is possible, but unusual.
The ambassador's statement was worded with typical diplomatic charm. Yet he was forthright in expressing his government's rejection of the UN resolution