The policies are detailed in MILPER Message NR 06-233, which may be Army specific. The biggest change, at least for the Army, is that one does not have to be in a language billet to receive the pay, and it has increased to $400 a month per language up to $1000 per month.
A quick Google search didn't turn much up, so here I've pasted the first part of the message. The whole thing is like 20 pages long, I could email it to you.
UNCLASSIFIED
MILPER MESSAGE NR 06-233
SUBJECT: FOREIGN LANGUAGE PROFICIENCY PAY (FLPP) REVISION
A. TITLE 37, UNITED STATES CODE, SEC 316.
B. NATIONAL DEFENSE AUTHORIZATION ACT (NDAA) FOR FY 2005.
C. DEPARTMENT OF DEFENSE INSTRUCTION (DODI) 7280.3, SPECIAL PAY FOR FOREIGN LANGUAGE PROFICIENCY, FEBRUARY 23, 2000.
D. AR 611-6, FOREIGN LANGUAGE PROFICIENCY PAY, CHAPTER 6.
E. MILPER MSG NR 04-185, SUBJ: FOREIGN LANGUAGE PROFICIENCY PAY (FLPP) CHANGES DTG 24 JUN 04.
F. MILPER MSG NR 02-162, SUBJ: FOREIGN LANGUAGE PROFICIENCY PAY (FLPP) DTG 22 MAY 02.
G. ALARACT MSG DTG 091642Z APR 02, SUBJ: CLARIFICATION OF FOREIGN LANGUAGE PROFICIENCY PAY (FLPP) MESSAGE DTG 111305Z APR 00.
H. ALARACT MSG DTG 111305Z APR 00, SUBJ: CHANGES TO FOREIGN LANGUAGE PROFICIENCY PAY (FLPP).
1. THIS MESSAGE WILL EXPIRE NLT 18 JULY 2008. HOWEVER, POLICIES EXPRESSED IN THIS MESSAGE WILL REMAIN IN EFFECT UNTIL A CHANGE IS MADE TO THE FLPP PROGRAM.
2. THIS IS A TWO PART MESSAGE.
3. EFFECTIVE 1 JUNE 2006, THE OFFICE OF THE SECRETARY OF DEFENSE AUTHORIZED AN INCREASE IN THE FLPP RATES. THE FOLLOWING REVISION SUPERSEDES CHAPTER 6, AR 611-6 AND THE MESSAGES REFERENCED ABOVE (E THROUGH H) EFFECTIVE DATE 18 AUGUST 2006.