What's the word for someone who can read/write/speak a language without understanding it?

If you have studied classical Arabic, yet cannot understand that language in spoken form, then you lack the skill aural comprehension. This is rather unusual, since as one of the passive skills, it's usually one of the first skills acquired. There is no word for this other than "someone who needs practice listening" if the skill is desired.

If, however, you can understand spoken classical Arabic, but would be utterly clueless if someone dropped you in Morocco or Bahrain because the local dialect diverges so much from what you can understand, then you could be termed monodialectical in Arabic.

One way of describing language competence which is not very competent is 'schoolboy French' or whatever.

David Cameron, who has notoriously poor schoolboy French, is urging today's youngsters to abandon the language of Molière and Voltaire to concentrate on the tongue of the future – Mandarin.

The Guardian

I don't know if this is related to medical terms; but if you're looking for it, then:

Wernicke's aphasia, also known as receptive aphasia, sensory aphasia or posterior aphasia, is a type of aphasia in which individuals have difficulty understanding written and spoken language.

Patients with Wernicke's aphasia demonstrate fluent speech, which is characterized by typical speech rate, intact syntactic abilities, and effortless speech output.

Aphasia itself is an inability to comprehend or formulate language.

Source: Receptive Aphasia