Question: In what language was the [first Zionist congress](http://en.wikipedia.org/wiki/First_Zionist_Congress) in Basel held?

Was it Yiddish, Hebrew, German, English? Where there translators?

Answer: 
[The Encyclopedia of the Arab-Israeli Conflict: A Political, Social, and Military History](https://books.google.co.il/books?id=YAd8efHdVzIC&lpg=PA1127&ots=OTYleCo7dP&dq=kongressdeutch&pg=PA1127#v=onepage&q=kongressdeutch&f=false):

> 
> The First Zionist Congress's official language, both spoken and
>  written, was German, but many delegates also spoke Yiddish
>  (Hebrew-German vernacular), the language of Ashkenazic Judaism, and a
>  Yiddish-like German known as *Kongressdeutch*.
> 
> 
>
Most delegates were Ashkenazi Jews, that is native Yiddish speakers (see the linguistic note below)... unless they were assimilated in German-speaking countries, in which case they would speak (a dialect of) German as their native language. They thus had no problem understanding each other, despite coming from countries with different majority languages.

Whether the language of Zionism should be Hebrew or German was a subject of a debate lasting for a few decades, with notably Herzl advocating for German, and [Technion teaching in German](https://en.wikipedia.org/wiki/War_of_the_Languages). [10th Zionist congress was the first zionist congress where as session was held in Hebrew](https://www.jewishvirtuallibrary.org/first-to-twelfth-zionist-congress-1897-1921).

**Linguistic note on Yiddish and Yiddish speakers**  

[Yiddish](https://en.wikipedia.org/wiki/Yiddish) is a *Germanic language* (more precisely a group of languages) - written in Hebrew script and with about 10-20% of its vocabulary borrowed from Hebrew, Slavic, and Romance languages. It is thus generally mutually understandable with German, to about the same extent as different German dialects are mutually understandable, or the German spoken in Germany vs. Swiss German/Alsatian.

An educated Yiddish speaker would typically speak Yiddish, the majority language of their country and Hebrew (which was learned as a part of the basic religious education, like Latin elsewhere, but was at the time of somewhat limited use for everyday communication). Even if they did not come from a German-speaking country, many would know German, since it came as an easy addition to Yiddish and, importantly, played the same role (alongside French) as English plays in the modern world.