In my conworld there's a special race of people which can perform magic (not that they're mages, but let's call them this way). The language they use for speaking is the same for spells and when one perform magic is like he's talking to his own spirit.

So the matter is more when you intend to make a spell or not. When you intend to ask your spirit to do something, then the magic is performed. The language itself is important, because is the only one understood by the spirit.

Theoretically, the non-mage humans also can perform magic, but their spirits don't have enough power to bear complicated spells (making them older or causing death). And humans usually don't know how to avoid the spirit admitting the conversation was directed towards him (is hard to control the "intention"). Mages do it naturally, though.

The thing complicate when arise a hybrid human-mage species and they have to control the intentions when learning the language, otherwise the results would be disastrous.

So, the language has words that MEAN things, it cannot be changed or the spirits would not understand (not a trouble for the immortal mages, but the hybrids have a hard time keeping things straight). As it is used for conversation too, it's not an imperative language by its own (there is an imperative mood, though), one have to make the spirit understand what you want (and as the time pass by, the spirit gets used with your way and lower level spells don't even need words).

Magical conlangs are pretty much tied up to how magic works in the conworld and how the people understand it.