This is a tricky question.

First of all, it depends about which time period of ancient greece we are talking. You can assume, that pre-Alexander greeks never had direct contact with indian people in greece, simply because there weren't any there.

But, they knew that there was an india, altrhough not specific details about it.

This comes because the Persian Empire (and it's different dynasties/ruling classes) served in the ancient world as a connecting link for the "western" and "eastern" world (China, India).

Besides the first impression, the persians and greeks were not at war all the time, so wares, coin and most importantly, information came to greece this way.

We also cannot rule out personal encounters of greek merchants in persia with indians.

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As for basic communication, this was either quite similar like today (Helping yourself with your hands and feet) or the merchants simply resorted to a third party language which was know to both of them. Or they used a translator. Again this is assuming we are talking about the "classic" antient period, where there was already an established infrastructure.

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Going as far as for the "first" merchants we get to a tricky zone. We have quite good documentation about the mesopotanian citystates and later the first centrelised kingdoms, which arose from this area, however as far as I know, first communication procedures between trading parties was not documented.

But, we can be sure, that communication was definitely possible at the end of the day. A buying/ selling process is, if you have the ware at hand to point on, actually not a big problem to communicate.

We can find clear influence of mesopotania in the first dynasties in Egypt, of even determine middle-european mined metals in artifacts found there.

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