While zhey Hrakkar is busy with his work, I think I'll give it a go and translate the song for some dothraki practice:
One night, I hold on you
Ooh, ooh, ooh, ooh, ooh, you
Castamere, Castamere, Castamere, Castamere
A coat of gold, a coat of red
A lion still has claws
And mine are long and sharp, my Lord
As long and sharp as yours
And so he spoke, and so he spoke
That Lord of Castamere
And now the rains weep o'er his halls
With no one there to hear
Yes, now the rains weep o'er his halls
And not a soul to hear
Now in Dothraki:
At jalan, anha qorak yera
Ooh, ooh, ooh, ooh, ooh, yer
Kastamir, Kastamir, Kastamir, Kastamir
Qemmosorkhiro hoshor, qemmosorkhiro virzeth,
Hrakkar zin mra qora arakh,
Ma arakhi anni neak ma has, Khali anni
Ven neak ma ven has ven yeri
Majin me ast, majin me ast
Haz Khal Kastamir
Ma ajjin eyel laqi oleth okrenegwini
Ma vorsakhoon rek memat
Sek, ma ajjin eyel laqi oleth okrenegwini
Ma vorsakhoon rek memat
I'm pretty terrible at demonstratives and locatives in dothraki so if I've made a mistake with those, let me know!