Two models of 'where spells come from'
An interesting question to ask of the collectively imagined meme of a spell or magical phrase is 'where did it come from?'. Why, for example, would a spell use human sounds? Why would it be made of the same phonemes we use in our speech? Why are all the magical words a bit like normal words, but not the same?
I present two models, which might help with back story for this (and for a well-known book and movie series). The first is that spells are programmed in by master witches and wizards, to be shortcuts for others. The second is that they form a specific magical picture in the mind of the person casting, which ties to the universe's magical field.
A magical universal field
Consider a spell where we have to hold a particular type of onion and utter the phrase 'Alliux!' to make someone we are picturing cry. In this model, the word itself would be essentially meaningless, but its purpose would be to recreate that particular thought from when we learnt this spell, for which the onion and the oniony phrase derived from allium (the family to which onions belong) serve as a mnemonic.
This model would support the idea that you must cast the spell very carefully, that it requires practise, and that ideally someone taught it to you well so that you can recall the state of mind exactly. It would also require the person casting it to be magical, as the spell word by itself is arbitrary.
Such a model might tie into dreams quite nicely; you might recreate some of those magical images in your sleep and accidentally cast them if you are having a fever dream, or are someone who sleepwalks.
Magic as a programming language with shortcuts
In this model, there is essentially a magical field accessed via either a very magical person or creature or via a spell they have created. In this model, the spell word is very important, because it represents the way to access the shortcut. The spell creator might have made it a global spell, or spells could gradually diffuse out from a specific place.
This would make it easy for non-magical people to invoke a spell, as long as they know the word. However, incorrectly invoking it if you aren't careful about the pronunciation, or if the invocation is complicated by requiring input parameters which are hard to simultaneously provide (e.g. it uses the thing you're holding or the name you think of, or any other contextual or mental parameter), then you may invoke it wrongly.
The best spells (like the best code) would fail gracefully if they detect incorrect inputs, or require a validation step (you cast the spell and then do something to confirm it, or an enchanted vision appears to ask you). The worst spells would just take your dodgy input and carry on.
This model has an excellent explanation for the words and language used for invocation; the invocation was chosen by the spell creator, and must not be accidentally spoken. So old spells would use old languages, or words similar to those in old languages. And foreign spells would use similarly foreign sounds. Mixing with people from a different heritage might permit you to learn their language, and some of their spells, if you could only make that glottal sound.
In programming, one of the difficulties is invoking some code using an object you made elsewhere, i.e. combining one codebase with another. Even when very careful, this can cause lots of unintended side effects; the object doesn't have the expected properties or isn't set up in the way the code expected, so the code ends up in an unexpected state. This could be a useful mechanism for accidentally making cursed objects; an ancient Arabian oil lamp being used to invoke a European storage spell accidentally produces a one way tardis which sucks in anything that touches the spout until rubbed by a worthy child.
Another particularly egregious problem could be that someone knows a privilege-escalation spell, and keeps making undisciplined people into spell-creators. They in turn keep making awful spells, polluting the vocabulary of normal speech with accidental invocations. People who lived near such a historically cursed place would have a huge range of shibboleths and unutterables, whole sound groups to be avoided for fear of unleashing irritating spells.
The best spells would in fact be hard to conjure, requiring very specific invocation with very specific objects, to ensure they were not cast accidentally, and to make sure the parameters are indentifiable. This would give rise to things like potions in which particular meaningful items like a lock of hair are dropped at particular points; this is both safeguarding the invocation and identifying the target of the spell.