1) Regarding the question of the members of Sanhedrin knowing 70 languages:

a) Tosfos (DH v'Yod'im) cites the Gemara in Sanhedrin (17b) which states that one may not convene a Sanhedrin in a city which does not contain two people "to speak" and one person "to hear." Rashi explains that this means that a city needs two who can speak 70 languages and one who can understand them, even though he cannot speak them. The Gemara there also cites a Beraisa that if there are four people on the Sanhedrin who know 70 languages, there is nothing greater than this.

b) Tosfos here writes that according to this, when the Gemara states that all members of the Sanhedrin know 70 languages, this does not mean that literally all of them know the languages. Rather, it means is that some of the Sanhedrin must know the languages.

c) The Rambam does not mention explicitly the 70 languages. Rather, he writes (Hilchos Sanhedrin 2:6) that the Dayanim must know most of the languages, so that they should not need to take resort to a translator. The Kesef Mishneh writes that the Rambam wrote that they must know most languages because it is very rare to find someone who knows all 70. The Rambam also writes that one should "make an effort" that the judges possess this quality. The Kesef Mishneh explains that the Rambam understood that if one can obtain such individuals one must do so, but if not, the Sanhedrin is valid even without them.

2) Now to the question of how many different languages Jews spoke in those days:

a) We find that Nechemyah rebuked the people (at the time of the Babylonian exile, after the destruction of the first Beis ha'Mikdash):

"I saw that the Jews had taken wives from Ashdod and Amon and Moab. Their children spoke half Ashdodit and did not know how to speak Yehudit but spoke according to the language of every country" (Nechemyah 13:23).

b) The Rambam (Hilchos Tefilah 1:4) writes that this was the reason why a fixed Nusach was introduced for the Shemoneh Esreh:

"When Yisrael went into exile in the time of the wicked Nevuchadnetzar, they mixed with the Persians and the Greeks and other nations and children were born to them in the lands of the nations. The languages of those children became mixed. Everyone's language became confused from several different tongues. People were no longer able to speak in only one language and many mistakes crept in. As a result, when anyone wished to pray and ask for his needs or praise Hash-m, he was no longer able to do this in the Holy Tongue without other languages also being introduced. This is why Ezra and his Beis Din instituted the Eighteen Blessings."

c) The above relates to the Galus Bavel, but it was only a limited number that returned in order to rebuild the second Beis ha'Mikdash. Rashi (Kidushin 69b, DH v'Avinah) writes that those who dwelt in Bavel "were secure, while those who returned to Eretz Yisrael were poor and had to work hard and were under threats from all their neighbors."

d) This is not by any means a comprehensive study, but the above sources can help us understand why a lot of different languages were spoken by Jews in the time of the second Beis ha'Mikdash.

Kol Tuv,

Dovid Bloom

Here are additional sources which illustrate that 70 languages were not literally required by a Dayan on the Sanhedrin:

1) I cited above the Rambam (Hilchos Sanhedrin 2:6) who says that the judges "must know most of the languages." The Lechem Mishneh writes that even though the Gemara states that they must know 70, there is a suggestion in the words of our Mishnah, which says that Mordechai "Yode'a b'Shiv'im Lashon," that knowledge of the majority is sufficient. The Mishnah does not state "Yode'a Shivim Lashon," but rather "Yodeya b'Shiv'im Lashon." If it would have stated "Shivim," this would have meant that they must know 70. Since it says "b'Shivim," this means that it is sufficient to know "b'Miktzas Shiv'im," or "some of the 70."

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2) The Aruch ha'Shulchan (Choshen Mishpat 1:24) takes this further and writes that knowlege of most of the world's languages is very difficult to find in one individual. If we would say that the Sanhedrin is disqualified without this, we would never have a valid Sanhedrin. It appears that Mordechai's knowlege of languages was something extraordinary. When the Rambam writes that they must know most of the languages, even though the Gemara states 70, this means that in the days of Chazal these were the current languages in their countries.

3) Finally, back to the question of how many languages were spoken by Jews in the time of the second Beis ha'Mikdash. On the Mishnah in Shekalim 5:1, that says that Mordechai was responsible for the bird offerings brought by women who had given birth, the Tiklin Chadetin writes that all of the women who brought bird offerings spoke different languages according to the different countries from which they came. One learns from this that Jews came to the Beis ha'Mikdash from lots of different countries, with many different tongues.

Kol Tuv,

Dovid Bloom