The Gostak is a game partly, but not entirely, in English. The usual word order words apply. Most pronouns, conjunctions, and prepositions are unchanged ("you", "and", "on", etc.), and some modal verbs ("can", "could"), as well as a few other words somewhat arbitrarily ("five"). But most of the adjectives, nouns, and non-modal verbs have been swapped for their own new vocabulary.
The result is a game you have to decode in order to play -- or decode by playing. Often the best way to work out what kind of thing a word represents is to try interacting with it. As you go along, you'll likely find yourself building up a little lexicon of common terms, and gradually room descriptions and command responses that seemed incomprehensible slide into making sense. It's a slightly eerie experience.
The linguistic complications extend even to the help menu, which is also in the Gostak language. This might sound annoying, but in practice it's very useful: if you know how IF games usually work, seeing typical help text translated into Gostakese is extremely enlightening.
Which brings me to my next point. The Gostak is not the game you want to start with if you're new to interactive fiction, because a lot of the decoding process is aided if you already know what commands are typically used in IF and what kinds of things the model is likely to include. But if you've played a few IF games and are looking to have your mind bent in a new way -- a way that would be impossible to imagine in anything but a textual gaming medium -- then you might want to give it a try. Expect to spend a lot of time on each turn making dictionary notes, even for the simplest of moves...