No doubt that the work of the translator is very important. When the translator does his job sincerely and honestly, he will be doing a great task. Moreover, the work of the translator is lawful in that which is originally permissible to translate.
The European Council for Fatwa and Research, states the following: The work of the translator is lawful in what is permissible to translate, such as the statements of people to the relevant institutions; therefore, working as a translator can be very tricky and translators should not exert themselves to discover the truthfulness of these statements, for Allah does not order you to do so, and it is not among the duties of your job.

Therefore, as a translator, you may translate any statements made even if the speaker is not truthful. The most that may be required from you is that if you are sure of him telling lies, you may advise him, if that would not result in a worse outcome than that of telling lies and would not lead to a legal offense.
Similarly, if you are a private translator, you do not have to investigate the truthfulness of your clients, and you should not follow their defects or have suspicions about them. If you find out for sure that what you translate is a lie that would lead to usurpation of sanctified right, be it public or private, in this case it is impermissible for you to assist your client in that matter, for Allah Almighty says, (Help one another in righteousness and piety but do not help one another in sin and rancor. (Al-Ma’idah 5: 2)