I would assume the basis mangaka's use for witch incantations and the like would come from Mantras. sometimes they are called Arias or incantations but the basic principle is that the words that make them up hold value either to the actual spell or to the user.
In Soul Eater the witches use Arias which flow from their themes. For Blair, even though she's "a cat with a ridiculous amount of magic", her attacks are themed around Halloween and pumpkins are a symbol of Halloween eg. "Pumpkin Pumpkin, Halloween Cannon!".
For Medusa, her theme is Snakes, so when she says "Nake snake cobra cobubra", it's a reference to snakes ("Nake" and "cobubra" may be deliberate misspellings of Snake and cobra), and her Vector Magic seems to resemble snakes in how they move.
I haven't seen Negima however from nhahtdh's comment it would work out the same since the words "feels comfortable and natural to the user" the words would have a psychological effect on the caster. so most likely a new mage will have weaker/unstable spells in the beginning but as they practice they build up an Aria which will make them feel conformable.