"Senedd" is Welsh, not English, so "The Welsh Senate" should not be marked as incorrect
The section notes have just (26 Meh 2016) been updated to explain more about senedd and how it is used in Welsh. The English translations have also been brought into line with the updated Notes, although the changes may take a little while to filter down to the system and the app.
In the case of 'the Senedd', y Senedd, in Cardiff, the word Senedd is not translated into English - it is the name of the building in which the National Assembly meets.
Wales does not have a senate, just the single-chamber National Assembly for Wales - www.assembly.wales It is reputed that Wales did have a short-lived parliament in the Middle Ages. If we did have one now it would probably be called Senedd Cymru 'the Welsh Parliament' (a 'senate' is usually a university governance body, or similar, in British usage).
The word senedd on its own is simply 'a parliament ' or 'a senate'.