Headcanon- Allspeech (part two)
[[I already have another headcanon regarding Allspeech here but I figured out another bit while I was in the shower this morning so here it is]]
It takes time and exposure to a spoken language for an Allspeaker to pick up the written form. This is because the telepathic component of Allspeech can't really pick up on meanings from simple written symbols divorced from a speaker and their mind, which is full of the words' meanings and connotations and ideas.
But exposure to a living speaker brings more than just the words' meanings- it slowly builds an association between the words and syllables and sounds and the symbols that make up the word in its written form. This takes time, because most people don't really think about the written form of a word when they speak it. The Allspeaker just has to slowly absorb bits and pieces.
Likewise, a recording or broadcast, similarly dissociated from a living speaker, would be likewise initially difficult for an Allspeaker to understand if they do not already have a grasp of the new language.
Also, when attempting to write in languages that aren't particularly phonetic (I'm looking at you, English) an Allspeaker would initially have a lot of trouble with spelling, unless like Loki they are an obsessive reader and learn the spellings through brute exposure. A language that has less complicated spelling patterns, such as Spanish, would be easier, because the written form more closely resembles the spoken version.
Silent letters would be the bane of most Allspeakers' existences.