Here's a JAV dictionary i found a while ago. It wont help with complex conversations, but it's nice to know more or less what's going on. Beware, long post:
atatakai - warm
atsui - hot (temperature)
bakku - doggystyle
bero - tongue
bikkuri - surprised. Girls often say this when a guy whips out his schlong and its way larger than she expected.
biku biku - throbbing (referring to dick)
bin bin - hard (referring to dick)
bukkake - literally means "splashing". Often used in JAV to mean cum on face.
chichai - small / tiny
chikai - too close.
chikubi - nipples
(o)chinchin - penis. Semi-slang. English equivalent would be "your pee-pee"
(o)chinpo - penis. Semi-slang, more vulgar/slutty than chinchin. English equivalent would be "dick" or "cock"
choudai - give it to me. Often heard in bukkake, the girl will say ippai choudai = "give me a lot [of cum]"
chu - kiss (slang)
daisuki - I love it/I love this
dashite - release / put out. Ex: ippai dashite = release a lot [of cum]. bero wo dashite = put out your tongue
dekai - huge (referring to dick). Bigger than ookii
dekachin - huge dick
deshou - don't you agree? / am I right?
dete kita - it came out
dopyuu-dopyuu - describes the sound of continuous spurting thick liquid, often said during bukkake
doro-doro - again, describing thick liquid flowing
doushio - what should I do?
ecchi - perverted / dirty. Japanese pronunciation of the letter "H" which stands for hentai which means "pervert"
eroi - erotic / dirty. Comes from the English word "erotic" and means the same thing.
fera - fellatio/blowjob
futoi - thick/fat
gaman dekinai - I can't bare it anymore
gaman jiru - pre-cum
genki - energetic / lively. Often referring to penis when it is very hard or throbbing.
goshujin-sama - master. Girl will often say to a man when wearing a maid costume.
haitta - it entered / it's inside. Often said when something goes inside pussy such as penis or vibrator.
hayai - fast / quick. Said when a guy ejaculates prematurely (cums to quickly).
hairanai - it won't enter / it won't fit.
hairukana - please enter
haiyaku - quicker / faster.
hajimete - first time. Girl will often say when she has never done a particular sex act before.
hazukashii - ashamed / embarrassed / shy.
hen - weird / strange
honto (ni) - very / really / seriously.
As adjective Ex: honto ni ookii = really big
As a question Ex: honto ni? = really?
hoshii - (I) want
ii - good / is it good?
ikasete - make me cum. Girl sometimes says when having sex
ikisou - I'm going to come
ikitai - I want to come
iku - come / I'm coming
imouto - little sister
ippai - a lot.
irete - put in (command)
irete ii - is it okay to put in?
itadakimasu - expression used before partaking in a meal. You often hear a girl saying this before they receive a guy's load in their mouth.
itai - it hurts
jozu / jouzu - skilled at
kakete - pour (command). In JAV this basically means "cum on my face".
katai - hard. Often used to describe hard penis.
kimochii - feels good / does it feel good?
kimochii yokatta - that felt good (past tense)
kintama - balls / testicles
kitsui - tight, intense.
kirei - beautiful
kocchi mite - watch me / look at me / look here.
kocchi muite - face this way.
kochira - this way / this direction.
koi - thick
koko - here
kowaii - scary. New idols sometimes say this when its their first time doing something extreme
kuchi - mouth
kuchi manko - literally "mouth-pussy" often heard during deep/fast fellatio
kuchi ni dashite, kudasai - either "please come in my mouth" or "please can I come in your mouth" depending on who is saying it
kurushii - it's painful / is it painful? Not as painful as itai. Girl often says when trying to deep throat.
kusai - smelly / stinks / it stinks
kusuguttai - that tickles
mada / mada dame - not yet. Girl will often say when giving blowjob/handjob to tell the guy she doesn't want him to cum yet.
matte - wait
(o)manko - pussy
mazui - not good / unsavory
misete - please show me
motto - more
mou ikkai - once more
naka - inside
naka ni dashite - cum inside me / release inside
nakadashi - creampie
namete - lick (command)
namete mo ii / namete ii - is it okay to lick? Girl usually asks before giving blowjob
nigai - bitter.
nomu - drink.
nonda / non jatta/ nomimashita - drank / I drank it. Often used after swallowing cum.
nuru nuru - wet
oishii - delicious. Often used to describe the taste of cum.
oku made - put in all the way. Guy will say when he wants the girl to deep throat.
onani - masturbation
onegai - please
onii-san / onii-chan - older brother. Onii-chan is cute way of saying.
ookii - big
oppai - breasts
paipan - shaved pussy
paizuri - tit fuck
pantsu - panties / underwear
pero pero - lick / tongue flicking. Can also be said in a lower tone bero bero
saikou - (it's/that's) the best
saki / sakipo - tip (referring to penis). Often heard during handjob.
sawatte - touch
(o)shiri - buttocks / butt
shitai - (I) want to do it
shitsurei shimasu - allow me to bother you. It's a phrase used to excuse yourself before doing something.
shoppai - salty. Often used to describe the taste of cum.
soko - there
sugoi - amazing (when used alone). Can also mean "very" in combination with other words
suki - like
taberu - to eat
takusan - many, full
tama / tama tama - balls / testicles. cute way of saying
tattete - stand up. Sometimes used to describe erect (standing) penis.
ugoita - it moved. Girl often says when a guys dick twitches/jumps.
ureshi - I am happy
wakannai / wakaranai - I don't know
yamete - stop it
yarashi - dirty / naughty / obscene.
yasashii - gentle / kind
yukkuri - slowly
zen zen - completely / total
zenbu - everything / entirely