Who served as Japanese-language interpreters for US forces in WW2? Were they native speakers (Nisei, etc) or white servicemen trained to fluency in Japanese?

The overwhelming majority were Nisei. A training program for military translators was started in November 1941, and of the first group of 60 students, 58 were Nisei. The training program aimed to improve their Japanese language skills, and to teach Japanese and English-language military terminology, map-reading, some cryptography, and translation work. 45 of this first group of students graduated at the end of the 6 month program, and 35 immediately went to the front (to Alaska and Guadalcanal). Over 6,000 Japanese Americans (mostly Nisei) graduated and served as military translators, with over 3,000 working in Australia (with about 2,000 of them in Brisbane). Over 3,000 more Nisei were making their way through the training program on V-J Day.

While most served in the rear, significant numbers served on the front lines, assisting with interrogation of POWs and communicating with enemy soldiers and civilians to encourage surrender. Some even served behind enemy lines, with Merrill's Marauders and the Chindits, and with the OSS working with anti-Japanese guerrillas. Okinawan speakers were specifically sought to support the invasion of Okinawa (specifically to speak with civilians and Okinawan conscripts; regular Japanese soldiers would speak Japanese). 19 Nisei translators were killed in the war against Japan - far fewer than the numbers of Nisei killed in action in Europe, but still a high number for non-combat personnel who mostly served in the rear.

Prisoner interrogations often yielded excellent intelligence - Japanese doctrine was that Japanese soldiers would fight to the death rather than surrender, and therefore didn't need to be taught what not to say during interrogation.

While the Army was willing and eager to recruit Nisei as translators, the US Navy was not, and only trained white translators in its own training program. Unsurprisingly, this training produced far too few translators, and even fewer good translators. Nisei translators working for the US Army were attached to the Navy.

Needless to say, many Nisei working as military translators felt rather conflicted due to family and friends being interned, and by working against the country of birth of their parents, and also due to racist they encountered when loyally serving their country.