This is usually used with the article, so "y gwanwyn" = (the) spring.
I thought so, too, but that was not accepted for me just now.
A good idea, thinking about it, to have 'the spring' and y gwanwyn forms for the seasons as the default forms. I'll see what we can do.
I would prefer it if "spring" were translated as y gwanwyn and y gwanwyn as "spring".
Please, if possible, do not require "the spring" as a translation of y gwanwyn -- it may be closer to the Welsh structure but sounds unnatural to me in English.
At the moment it accepts both 'spring' and 'the spring', as both are common in English in various contexts.
Yep, "the spring" doesn't sound at all unnatural to me.