Note: this is an in-universe explanation, not media in general.
Media in general tends to have magic written in archaic, usually dead, languages. In my world, magic is best described as fickle; most known spells are "iffy" with many having unintended consequences. For example, a spell to cure a lame limb might also turn it ghostly pale or a spell to cure one's cow might cause it to also grow another tail.
The spells themselves are written on tablets, scrolls, etc., passed down from generation to generation within the 'Mage Families'- people who bought spells in the market, happened upon an ancient library or wizard's tower, or accumulated them over the years.
While new copies of spells can be made, only a few select spells are seen as worth copying due to them having been used enough times to have a clear outcome and side effects. However, the spells can only work if they are transcribed exactly as written, which means writing in the ancient "Magic Languages".
What would be a good in-universe justification for the "Magic Languages" to be a series of dead languages while contemporary languages are unable to have any ability to make spells work?
Note: by dead language I mean no one speaks it as their native language, and the Mages themselves have a very minimal understanding of it.
In this world dreaming is a very important part of the Theology so tying in dreaming would be nice, but isn't necessary.
Brand new spells are very, very, very hard to create with only a few being made every hundred years or so. However, finding new spells is rather common.