Translation:In the synagogue they speak only in Yiddish.
I was marked wrong for 'In synagogue they only speak Yiddish.' which seems like a reasonable translation to me. In English, it's not necessary to say 'in' Yiddish. Also, the definite article isn't completely necessary in English. Just as you can say 'in church, not 'in the church', you can say 'in synagogue.' (There is a subtle difference between the two depending on context).
Yes, רוזלין (rBhr5), your answer seems reasonable to me, too. However, it changes the grammar and you agree that it changes the meaning. Since the more literal translation is also a good English sentence, don't you think that it's also reasonable for them to reject your answer (as they did for me, too)?
(Edit) My results today:
In synagogue they speak only Yiddish. REJECTED
In synagogue they speak only in Yiddish. REJECTED
In the synagogue they speak only Yiddish. ACCEPTED
Actually I always wanted to ask, חרדים, do they use Hebrew in everyday life? If not then which language do they speak? I'm pretty sure I read somewhere they regard Hebrew as a holy tongue and prefer not to speak it. On the other hand I'd bet I have heard some of them speak Hebrew.
Yiddish is the the language of Ashkenazi Jews. The same way Ladino is the language of Sephardi Jews. There are many different Jewish languages that are a combination of the local language/s and Hebrew... Judeo-arabic, there was one in Greece, and Italy, etc.
Yiddish has two main versions that are mostly accents/pronunciation depending on where one lived - Litvak (Lithuania, Germany, Poland) and Galitzianer (from Galicia, part of then Austria, now Poland and Ukraine). There are other versions, but these are the main ones. https://www.worldatlas.com/articles/what-are-the-jewish-languages.html
You can hear a song with the two versions of Yiddish: https://youtu.be/PTVVJ2uxPcI