ReMe Level 5 guideline is "Control a disembodied spirit."
Do you have to speak a language the spirit understands to issue commands (e.g. if controlling the ghost of a Welsh warrior, do you need to speak Welsh)? Or are commands somehow made clear through the nature or your magical control?
The Mentem basic description and Intellego Guidelines in ArM5 appear to say nothing about language.
I'm going to guess it's on a saga by saga basis as per the Storyguide's opinion.
Given the overall nature of language barriers in Mythic Europe, I personally would go with: to read thoughts or speak with spirits, the caster and target do need to share a language. (Now watch someone quote some rule somewhere saying something else.)
+1.
Well, InMe Level 5 guideline is understand the meaning behind spoken words - which suggests that spoken speech and mental thoughts are differentiated in some fashion and that speech is not immediately intelligible.
The question in my mind is whether speech is necessary to command a spirit controlled by Rego magic or whether instructions can be communicated by thought (i.e. without the need for translation). I'm divided on this issue.
In the ReAn guidelines, commands given to beings by the medium of magic do not need to be understood.
Effects which make the target obey orders which are given afterwards still need the target to understand to obey.
So,
"Command the Harnessed Beast" (ReAn30) - gives a direct command, like find and kill a squirrel and return its body to me, does not require the target to understand. Equivalents could theoretically be made for spirits, with varying guidelines.
"Coerce the Spirit of the Night" (ReMe20) - makes the spirit pliant to commands, which must be understood to be obeyed.
Rego Mentem effects do not create immediate automatic mental control. The "commands" must be communicated, somehow, and in an intelligible language to the target.
If the mage can "put thoughts" (not just "words") into the mind of the Target, then that's ample. If the spirit happens to understand the mage's words, so much the easier, but (most) spirits have no "universal translator" with this regard. (Some spirits might read minds - if they can penetrate Parma, that covers it as well.)
Intellego effects give "information" to the caster. InMe spells should not need to worry about a language barrier, because they are reading "thoughts", not "words", whether in living minds or in spirits. (InCo(Me) Whispers thru the Black Gate should, imo, be much different than described, but that's a diff discussion.)
One the other hand, a spirit that understands humans generally but not their languages (such as a ghost) might well be controllable for simple actions with gestures and mimes. Although there is scope for misunderstanding, if the spirit does actually understand, he cannot fake not understanding as a get out.
I can easily imagine a magus commanding a ghost to kill someone in the vicinity (points at target while drawing his finger across his throat), come closer (beckoning), go away (shoo-ing him or pointing assertively).