How significant and accessible is modern Chinese astronomy? Is China fully integrated in the international astronomical community, or does military secrecy and language problems and sanctions (ITAR) or something else stand in the way? Does China have valuable assets for astronomical observations, and a vibrant national astronomical academical community?

China has an ambitious and successful HSF and Lunar program with orbiters and a lander. They have also announced a very ambitious program for inter-planetary probes to practically the entire Solar system, to date they've flewn by the NEO Toutatis. The are also building a 500 meter radio telescope super-version of Arecibo.

Will for example US astronomers be able to use that telescope? Will they be offered samples of the upcoming Chinese lunar sample return, maybe from the lunar far side? Have lunar ground penetrating radar data from Chang'e 3 been presented publicly and led to papers in international journals? Is there any prominent Chinese astronomical journal?
China is pretty well integrated into modern astronomy -- there are regular international conferences held in China (the most recent General Assembly of the International Astronomical Union was held in Beijing in 2012), lots of Chinese astronomers at other international conferences, Chinese participation in international observatory projects (the Joint Institute for VLBI, the Thirty Meter Telescope, the Square Kilometer Array, etc.), and so forth. I'm not sure I'd say they have any really significant facilities yet (with the possible exception of LAMOST), but they have very ambitious plans, as you've noted.

(ITAR has very little to do with astronomy except when it comes to a few things like nuclear physics codes produced by places like Los Alamos.)

I don't know how much international participation is anticipated for the FAST project, though I gather the instrument development has some involvement from British and Australian institutes, so they'd likely get some time.

Yes, results from the Chang’E-3 mission have been published in scientific papers, e.g.
http://www.sciencemag.org/content/347/6227/1226.abstract

There are several Chinese astronomy journals (e.g., Acta Astronomica Sinica; Research in Astronomy and Astrophysics), but like most "national" journals they aren't that important internationally.