As mentioned above, there aren't any general prohibitions (per se) on translating Jewish texts into the vernacular, be it English, French, Russian, Yiddish, etc.
Regarding Text on This Site
Many posts and answers here are among those for whom Hebrew is not a foreign language. Many of this site's users know each other (at least virtually) and are aware of each others' levels of knowledge. Things left untranslated are usually done so not to lock others out, but instead to save time. If I know that user A readily understands Hebrew, I might not bother translating an entire passage to English in the interest of saving time.
Feel free to comment on questions and answers which leave texts untranslated to prompt the authors to translate them for the world at large.
Regarding Entire Jewish Texts
The main reason something might not have been translated into the vernacular is usually based on demand and desire for said title. For example, you probably won't be able to find Yosef Karo's Shulchan Aruch translated into English. You probably also won't find the Rema's commentary to the Shulchan Aruch translated into English. However, because there is a big demand for understanding contemporary Jewish law, you'll find a translation of the Mishnah Berurah (which probably contains the original quotes from Shulchan Aruch and the Rema's commentary) and the Shulchan Aruch HaRav (which also probably contains the original sources translated).
The companies that translate these titles operate on supply and demand. Gemara is a primary text, so there are more than one translation of it widely available in English. The five books of Moses are also very primary, so you'll find many translations available there.
When you get to certain titles, e. g. Etz Chaim and the Zohar, much knowledge about other Jewish literature is assumed to be within the reader's grasp. A good example of a similar text translated is the Likkutei Moharan, the magnum opus of Rebbe Nachman of Breslov. Breslov Research Institute finished translating the entire sefer, which in Hebrew is around 200-300 pages, only a couple years ago. The project took 30 years, and not for lack of funding. Since Likkutei Moharan assumes knowledge in Tanach (Torah, Neviim, Kesuvim), Rabbinical literature (Midrashim, Gemara, Yerushalmi, etc.), Halacha (Rishonim, Shulchan Aruch), and Kabbalah (e.g. the Zohar, Etz Chaim), simple phrases in the Hebrew require vast amounts of commentary to explain their meaning.
In this example, if you were to simply possess the ability to read Hebrew fluently, you would still find Likkutei Moharan to require enormous amounts of contemplation to understand. Though many things are made evident in the sefer and it's easy enough to follow along, you'd be missing the context of the sefer in light of everything else.
In summary, a title may not be translated and readily available in the vernacular for the following reasons:
- There isn't enough demand for it, i.e. there are more pressing Jewish texts to immediately translate.
- It would require an enormous amount of work to translate while trying to maintain the idioms and context in which it was written.
For these two reasons, a lot of onus is put on the reader to do his best to begin learning Hebrew and Jewish texts, as it opens doors to the entire world of Jewish literature, rather than individual doors to individual works. For some people, learning Hebrew is exceedingly difficult, and in that case, readers will have to await (and demand!) translations of the texts they wish to study.