Where English is an official language, translation is not necessary to satisfy local rules. However, the inquiry does not end there.
In most cases involving service of process in countries where English is not an official language, two concepts require translation:
First, any effective service must comply with the laws of the receiving country — and most countries require that documents to be served are accompanied by a translation into that country’s official language.
Where local rules are circumvented and a translation is not supplied, service can be quashed in the forum court, and enforcement of a judgment in the foreign court may fail because the plaintiff failed to respect that court’s procedures.
Second, US notions of Due Process require that the defendant be able to understand the documents served.
Accordingly, a summons and complaint may require translation into more than just one other language.
An example: For an Italian defendant who does not speak English, but lives in China, translation into Italian will be necessary to satisfy Due Process concerns, while translation into Mandarin will be necessary to comply with local rules. Such hypotheticals require very careful analysis.