Learn Croatian: Savršeno govoriš engleski! - You speak English perfectly!

It uses the neuter singular form of the adjective! The base adjective is savršen (perfect). To turn it into an adverb meaning "perfectly" to describe how someone speaks, you just add the neuter ending -o, making it savršeno.

Is engleski technically an adjective or an adverb in this sentence?

Even though it looks exactly like the masculine adjective for "English", it acts as an adverb here! When paired with verbs like govoriti (to speak), language words ending in -ski describe how you speak (literally: "in an English way"). Because it acts as an adverb, it won't change its ending here.

Why isn't the language name engleski capitalized?

In Croatian, the names of languages are never capitalized! You only use capital letters for the names of countries (like Engleska) and the inhabitants of those countries (like Englez).

Could I change the word order and say Govoriš engleski savršeno!?

Yes, Croatian word order is very flexible and that would be perfectly correct. However, placing the adverb savršeno at the very beginning of the sentence adds a lot of emphasis to it. It makes the compliment feel stronger!