Formally, the language proficiency test is not subject to any prerequisites. This means that - regardless of his pilot ratings - anyone may go in for the language test and have the result certified.
Due to the the fact that special questions (ICAO spelling alphabet, reading back of clearances, aviation-related vocabulary) are asked, I do not recommend making the test prior to having a radio certificate. Even excellent native English speakers might not be in the position to answer certain questions without prior knowledge and would thus be rated below their respective level of knowledge.
