This charm makes a humanoid creature regard you as its trusted friend and ally (treat the target's attitude as friendly). If the creature is currently being threatened or attacked by you or your allies, however, it receives a +5 bonus on its saving throw.
The spell does not enable you to control the charmed person as if it were an automaton, but it perceives your words and actions in the most favorable way. You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn't ordinarily do. (Retries are not allowed.) An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing. Any act by you or your apparent allies that threatens the charmed person breaks the spell. You must speak the person's language to communicate your commands, or else be good at pantomiming.
[emphasis mine]
How, exactly, does one communicate one's commands to a charmed creature when there is no shared language between the two? I have seen this resolved using the Bluff and Sense Motive mechanics for sending and discerning secret messages, but I can find no actual support for that in the rules. Ranks in Perform: Pantomime would seem to be useful, but again, I can find no clear method for resolution in the rules.
Are there explicit rules for communicating commands to the subject of a charm person when the caster does not speak the subject's language (or the caster and/or subject speaks no language)? If not, how are you resolving such situations?
Not that I'm aware of.
I'd probably rule it as a Linguistics check, representing time spent observing the speaker's tongue and attempting to piece together simple words and gestures. See any episode of Stargate SG-1 whereby the team find themselves amongst a population whose language they cannot speak. Daniel Jackson, the linguist, always finds a way to communicate by sitting down with the natives and hearing them speak (usually by enjoying a meal by their side).
If the target of the spell speaks no language at all, it could come down to a Diplomacy check (similar to a druid's "Wild Empathy", assuming the target has a sufficient Intelligence score).