Language is essentially nothing more than a mode of expression. In spell work language fulfils the role of the expression of intent. My view on this would be that spells can be conducted in any language but would be best achieved by being spoken in the language that the caster is the most comfortable with. Ideally the language that person thinks in.
When I am putting together a spell, the intent can often be complicated enough to get correct in my native language. I call my apprentice our “Occult lawyer” as she is always the one who looks for the loop holes in the spells we work and what potential unwanted consequences we may be inviting. I’d say that casting in a language you are only partially familiar with could be a real recipe for disaster. Some languages certainly sound more magical and atmospheric than others but from a practical point of view, to me at least, it makes sense to perform magic in a language you know