The Japanese don't usually use gods for emphasis, so I'm replacing it.
Taihen! Ketsu ha suika ni taberarechatta.
Emphasizes your distress that you ass was eaten.
Taihen! Suika ga ketu wo tabechatta.
Emphasizes the improbable watermelon.
Consonants as the English (to a first approximation)
Vowels:
a as in Sheep says baa
i as in squeak
u as in Boo
e as in Canadians say 'eh'
o as in Oh god, a watermelon ate my ass.

I can provide with Slovenian translation:

Oh moj bog, lubenica je pojedla mojo rit!

and German
O mein Gott, die Wassermelone hat mein Arsch gegessen.

Hope that helps.

Alternatively, you can give the watermelon a streak of rabidity by alternating "gefressen" with "gegessen" in the German translation, since fressen is connotated with animals eating. Kind of like the difference with "eat something" and "feed on something". Plus there is just something funny about picturing a watermelon with cute little fangs.
You can also change the emphasis from straight forward to a bit slangy with:

Ach, Du lieber Scheiss, 'ne Wassermelone hat mein Arsch gefriss'!
[ahk do lee-ber shice(long I sound), nuh vah-ser-me-low-nuh hat mine r-sh guh-fris(short I sound)]

just throwing in my 2 cents.

Ach, Du lieber Scheiss, 'ne Wassermelone hat mein Arsch gefriss'!
LOL!!! Yeah, you are right, that does sound more interesting!!!
Just little thought - grammatically it is meinen Arsch oder?.
(I made a mistake too...., sorry,..)

umm, using dialect-ish type slurring (that's what I was doing in my head) I forgot the apostrophe on the 'mein' bit. I have to admit my grasp of the correct declensions is sketchy. It's been a while and I learned the language mostly phonetically, and got my butt beat up by my professors when started taking actual classes. I have a tendency to slur over everything, since I speak a dialect, and not really any form of Hochdeutsch. Which is fun, but not necessarily grammatically correct. And stuff. Umm, so yep.

got my butt beat up by my professors *G* YEAH!! I know exactly what you mean!!!
and then sometimes, I drove them into despair, haha!!

I have a tendency to slur over everything Mee too!!! Those cases are really pain in the ass. My language (Slovene) has it too, but really doesn't do me much good with German. I've been trying to improve my German to a proficient level - but in my heart I don't think it'll happen - ever, heh.

Which is fun, but not necessarily grammatically correct.
And fun we should have !!! grammar is boring anyway (necessary sometimes, though, but boring...bleh).

Sorry, I didn't mean to be pain in the ass myself, but I just thought if they are going to use it somewhere it might be good to get it right, even in slang. ( I am rather curious for what ever purpose this is?!) (Also: Deutche Sprache schwere Sprache - it took two of us to get it right *g*!!)

So then, cheers!

sometimes, I drove them into despair, haha!!
LOL, I drove mine crazy :o)
Sorry, I didn't mean to be pain in the ass myself
No, no not at all! I think if you're going to have fun with something, go all out and really play with it.

Hi, I hope this helps:

In Mandarin:

Tian(1) ah(4), wo(3) pi(4) gu(3) bei(4) xi(1) gua(1) chi(1) le(4)!
Heavens, my ass has been eaten by watermelon!

Numbers refer to the 4 tonal variations in Chinese:
(1) is a high tone, as in “HOW long must I wait for SV season III?”
(2) is a rising tone, as in “You waited HOW long?”
(3) is a tone that falls first and then rises. It goes lower than (2) and then rises in tone.
(4) is a sharp falling tone, as in “How can I wait till June? HOW?”

Sounds you need to know:

A is like “aah”
E is like “er”
Ei is like “ay” in bay
I is like “eee”
Ia is like in “yen”
O is like “aww…”
U is like “ooo”

And in French:
Mon dieu, une pastèque a mangé mon cul.

Swedish: "Herre gud, en vattenmelon åt min rumpa!"

I could choose a more vulgar word for ass if you want, but this is an accurate translation. How to describe the pronounciation...? All the letters are spoken; the e:s are like the e in the English 'end'; the å is spoken kind of like the o in 'lord'; all the words have the emphasis on the first syllable, except 'vattenmelon', where the emphasis is partly on the first syllable, but mainly on the last, which should sound vaguely like the English 'loon'. Oh, and 'gud' has a long vowel, but the Swedish u-sound doesn't exist in English, so it's hard to explain...um...imagine Ian McKellen saying 'Saruman', and then make the u longer, that should get you an approximation.

Or you could just take a leaf from the Swedish Chef's book. *g* That's what we sound like.

In modern Greek, it's OH ο Θεός μου, ένα καρπούζι έφαγε το γάιδάρο μου!

In Potugeuse, it's Oh meu deus! Uma melancia comeu meu burro!

And I think I'm totally mixing up the order this is supposed to be in, but if I'm not then the Lakota Sioux translation is:

Wan la wa! Mitawa pawaza wa spasniyutapi wote wa!

(Look! My ass a watermelon ate (female declarative)!)

Finnish: Herra jumala, vesimeloni söi minun perseeni.

Finnish is basically pronounced the same way it is written. The ö-sound is would probably sound something like the e is pronounced in the or the i in bird. That's the only way I can think to describe it.