The nursery is where language pairs go that can be built, but have not been extensively developed. They might be missing large portions of code, or some parts might be copied from somewhere else and not converted. It should take around 1—3 months work to bring the pair to staging/trunk level.
The basic requirements are that it should build without any external effort, contain all basic elements (rule files, modes) even if they're 'dummies', and function, for a very relaxed meaning of 'function' (word to word, generation marks allowed).
The nursery can be found here