Use a business letter format for the open components of your letter. Your name and address should be written in the top right of the letter. Beneath that, include the date, name and address of the organization, institution or individual to whom you are indicating your language proficiency. Use a formal salutation such as "To Whom It May Concern."
State your name and your reason for contacting the organization or institution. For example, "My name is Anderson Valhalla and I am writing to provide evidence of my proficiency in writing and speaking French."
Identify the appended evidence that proves your proficiency in a particular language. For example, "Appended to this letter is a copy of my college transcript, showing my successful completion of three college-level Arabic classes."
Explain how the appended evidence proves your proficiency. For example, "Successful completion of the intensive language program required written translation of short stories and plays, as well as full immersion in a Greek-speaking city for three months."
Close your letter according to business letter conventions. Thank the recipient for his time and consideration, use a formal sign-off such as "Sincerely," or "Respectfully," and sign your name.
Append your proof to the back of the letter. Legitimate proof of language proficiency could be a transcript or certificate of program completion.