"One of the diplomats is deaf."
Translation:Unu el la diplomatoj estas surda.
It's a rule of thumb because English isn't terribly consistent with its handling of these things. You can plausibly replace it with 'out of', and you have the likes of 'one out of five', which would translate as 'unu el kvina', which is a construction to 'one of the diplomats', but we don't usually use 'out of' in that circumstance, because 'diplomats' is plural rather than singular. If it was singular, then 'out of' would be quite natural, e.g. 'one out of the group' or 'one of the group'.