Tell, say, speak. All these words are used for verbal communication. They have some similarity but we choose one of them (and not the others) for certain contexts.
*We speak to someone when we are communicating officially such as during a lecture, addressing an audience, providing advice formally or talking to someone of a higher authority.
*We tell someone something when we are sharing some information or giving an instruction.
We say something when we want to express a thought, an opinion, a feeling or an instruction.
In your sentece:
A translator (1)______ the President what the Chinese visitors were (2)_____ .
(1) Assuming that the President does not understand Mandarin, the translator was interpreting what the visitors were saying before sharing with the President what he heard. Therefore, the translator was telling/told the President something.
(2) The Chinese visitors were most likely not speaking in some official capacity, since they were VISITORS. Though they could have "spoken" to the President face-to-face in a formal setting, there was nothing in the sentence to suggest so. Quite the opposite, these VISITORS were most likely expressing their thoughts casually. Furthermore, we usually say that we "speak ABOUT something". ABOUT is missing from the sentence.
If the sentence had been "A translator told the President what the Chinese DELEGATES ________ about during the meeting.", I would have chosen "were speaking" for the blank.
I hope this helps.