You always modify spells..you are at your best when you develop your own (obviously for that you can use samples, other's spells etc). Some say the spells best work when uttered in your own native tongue, but I do not subscribe to that. But I will say this, you better KNOW exactly what the words mean, before start using them...You can injure yourself if you use spells that are dubious or planted with mischief, that is when you have no idea of the meaning/and ramifications of the words used) I use English and Hebrew only (from my Quabbalistic pursuits)
It is also important to understand the three levels of magical speech. Saying out-loud is not enough. Imagine it, visualize it and say it with all of your being. No wavering, doubts etc.