The first part, "
Shekh ma shieraki anni," would work better with vocative particle in front of it: "
Zhey shekh ma shieraki anni." Dothraki vocative particle is mostly a syntactic device and usually left untranslated. If you want to visibly translate it, "
O my sun and stars" is close.
"Be the stallion that mounts the world" sounds quite clear to me: more engouragement (hortative?) than straight out order, but syntactically on the same direction as eg. oft Gandhi attributed "be the change you want to see in the world". I'd guess Dothraki informal imperative would do nicely. Other than that, though, the sentence happens to be rather tricky to translate, due to a certain peculiarity (copulalessness, that is) of Dothraki. I have no experience in languages other than Dothraki about zero copula sentences, so I'm a bit out of my depth. Sure I can create all kinds of interesting translation attempts like:
Nemo azhas meyer vezh fin saja rhaesheseres. ~ Allow/let yourself to be the stallion that mounts the world.
...but if anyone (Niqqo?) has a better intuition on this, it'd be interesting to hear suggestions.
As for what else the latter line might be, the possibilities are already quite countless.
http://wiki.dothraki.org/dothraki/Idioms_and_Phrases might give some ideas.
Neakates jahak yeroon ma dikates sajo yeri. ~ Your braid be long and your steed be fast.
...
Aneakanazates jahak yeroon ma adikanazates sajo yeri. ~ Your braid be the longest and your steed be the fastest.
