It can take years to learn to read, don't worry about it. She sounds totally normal.
Do you mean she's learning with a phonics based system but the books she is being given for reading aren't decodable with the sounds she's done? That's normal because phonics is fairly new and there is no reason for schools to throw away hundreds of perfectly good books just to replace them with phonics readers. It is a bit of a pain and would be better if all children had access to phonics readers, but it wouldn't be practical/sensible, so you have to make do.
What you can do is a couple of things. First practice blending with her where you really draw out the phonics sound (e.g. sssss like a snake, not suh, like someone else said) on words where this works well e.g. sssssssuuuuuuuuuunnnnnnnn - ssssssssssuuuuuuuu, uuuuuuuuuuunnnnnnnnnn, suuuuuun, sun. There are lots of blending activities online you can look at (check accent because many are American, they also classify vowels differently, so cross check with the phonics system your school is using.) The fact she's able to write words down by sounding them out is great, try getting her to sound out what she's written so doing it backwards.
Next when you read with her, whenever you come across a word she hasn't learned that spelling of the sound yet you should say "This is a tricky word, let's do it together." Cover up the word with your thumb revealing just the first sound(s) so she can sound this out, then add the "tricky" sound yourself and let her put them together. Some "tricky" words are high frequency meaning they come up so much children tend to learn them as sight words so they aren't held back. These will be words like she, then, the, all.
Are her other languages using the Latin alphabet too? If so this can cause a bit of a crossover with multilingual children. If she mistakes an English letter for the sound it makes in another language, you can say "That's right, w makes vvvv in German, but in English it makes the sound www, remember? So not vet, but..." If her other languages are more phonetic it's worth concentrating on English first, I found DS1 then transferred this knowledge and could instantly read in German with no effort from me.