The written language used by the changelings is quite easy to translate from Equish, with a fundamentally similar alphabet, and a nearly identical sentence structure. This is likely a result of early writers attempting to emulate the otherwise universally accepted language of ponies, griffons, dragons, and diamond dogs.
As a consequence of this mimicry, the written language of changelings is not the spoken language of Changeling, but a simplified version of Equish. Nothing is lost in translation, and some changelings do speak the same verbal form of Equish as the rest of the world, but it is not used outside of interspecies communication.
The alphabet is almost an exact copy of ours in the sounds produced by each letter, their placement in the words used, and somewhat simplified rules of grammar. For example, their is no equivalent for the letter K, as it has become indistinguishable from the letter C. Whenever a C makes an S sound in our language, the Changeling version of S is used, and similarly for the P–H pair being replaced with F.
There are Changeling letters that replace certain Equish consonant pairs, such as a single letter for the T–H sound, the S–H sound, the N–G sound, et cetera.
Certain vowels are also split into two separate letters, a long and a short form. An O with a silent E in Equish becomes a single letter.
This means that every possible syllable used in the written form of Changeling is a simple vowel–consonant pair.
This simplicity would be a great thing on its own, but I guess it was supposed be a consolation amongst the terrifying nightmare that is the actual use of this language. Most written documents are much more complex than a simple word for word translation.
Each letter is assigned a number value, and a single document can be recorded as a string of double digit values. When writing, a specially educated infiltrator will use a preselected keyword, such as the subject of the document they are transcribing, and enter the rest of the numbers into a special mathematical formula.
They then convert the resulting string of numbers back into letters, and then write that down.
The changelings encrypt their written word.
When a changeling then reads one of these documents, they are first decrypting it, then translating from Equish into the spoken Changeling language, which has no written form.
And they are doing this without so much as a calculator. Granted, they have a hive mind to work with, and offload the math to that, but still, I wonder why this standard of encryption was introduced to begin with. It seems like a lot of trouble to go through.