This case-insensitive code system http://hl7.org/fhir/us/directory-attestation/CodeSystem/LanguageProficiencyCS defines the following codes:

Code	Display	Definition
00	No proficiency	Unable to function in the spoken language.
10	Elementary proficiency	Able to satisfy minimum courtesy requirements and maintain very simple face-to-face conversations on familiar topics. A native speaker must often use slowed speech, repetition, paraphrase, or a combination of these to be understood by this individual.
20	Limited working proficiency	Able to satisfy routine social demands and limited work requirements.
30	General professional proficiency	Able to speak the language with sufficient structural accuracy and vocabulary to participate effectively in most formal and informal conversations in practical, social and professional topics.
40	Advanced professional proficiency	Able to use the language fluently and accurately on all levels normally pertinent to professional needs.
50	Functional native proficiency	Speaking proficiency is functionally equivalent to that of a highly articulate well-educated native speaker and reflects the cultural standards of the country where the language is natively spoken.