There may be more to this question than is initially apparent.
Clearly, a mediator needs to communicate effectively with all parties and their representatives. Where the parties do not share the same mother tongue, it may be useful to consider a mediator who can mediate successfully in the parties’ natural languages.
It is most important to ensure, however, that the parties can effectively communicate with one-another and with the mediator, and that understanding is facilitated. Using co-mediators, each with different language and cultural skills, may be one way forward. Another approach is the use of interpreters. If they are independent of the parties they can be considered aides to the mediator, and therefore have an important neutral role.
In addition to language capability, consider the mediator’s cultural acceptability to all the parties, including the mediator’s ability to engage effectively with the parties. Where there are language issues, there are also likely to be cultural issues, but there can be strong cultural differences among parties having the same mother tongue (e.g. where different professional cultures are involved).
Where there is a diversity of cultural backgrounds among the parties, a mediator or Mediation Provider that shares only one party’s cultural values and credentials may not be trusted or accepted by the other party. Consider expressing a willingness to select a mediator or Provider perceived as sharing the other party’s cultural background, or a mediator very familiar with both cultures, or propose using one from a third culture who is skilled in negotiating cross-culturally, or an international institution or provider. Again, co-mediators can be a solution. In situations where the other party shows reluctance to engage in mediation, these approaches can be a way to help them feel more comfortable with the process.
Above all, ensure that the mediator is sensitive to cultural diversity and has the capacity to understand the differences involved and implied by cultural dynamics.
Mediation relies for its effectiveness on all parties trusting the mediator. It is part of the mediator’s job to earn that trust.