What's the point of you posting these sentences? Yes, they are similar (but different) ways of saying the same thing. No, they won't be accepted because they are different sentences. There are usual a dozen ways to express the same thought, it wouldn't make sense to accept them all along with their many variations.
There's a thin line between adjectives and adverbs in English, but not in Czech - they are very separate in Czech.
In this English sentence, "worst" is an adjective ("nejhorší čeština"), but if you place it after "Czech", it becomes an adverb ("česky nejhůř"). The former option would be unnatural in Czech ("mluví nejhorší češtinou" - clunky), we only use the latter (using an adverb).
- Adjective: špatný; comparative: horší; superlative: nejhorší
- Adverb: špatně; comparative: hůře, hůř; superlative: nejhůře, nejhůř
- Adjective: dobrý, comparative: lepší, superlative: nejlepší
- Adverb: dobře, comparative: lépe, líp, superlative: nejlépe, nejlíp
Notice that adjectives always end in "-ý" or "-í" (hard vs. soft) in their basic dictionary form, i.e. masculine singular, and then they change the ending depending on gender, number, and case.
Adverbs, on the other hand, never change their form, they only have 1 form. They usually end in "-ě", "-e", or "-u" (i.e. vowels that adjectives never end in) - or, in case of shorter forms like "hůř" and "líp", a consonant.
Another comparison to study:
- He speaks English well (adverb) - Mluví dobře anglicky.
- He speaks good English (adjective) - Má dobrou angličtinu. (literally: Mluví dobrou angličtinou.)
- He speaks English better (adverb) - Mluví lépe anglicky.
- He speaks better English (adjective) - Má lepší angličtinu. (literally: Mluví lepší angličtinou.)
- It's obvious that English prefes adjectives in these sentences, while Czech prefers adverbs.
Thanks for that very comprehensive description of the Czech formats for adjectives and adverbs. I would recommend any student to take note. You are quite right about English having a fine line between adjectives and adverbs, English is a very sloppy language indeed! (we only have self appointed language police and nobody takes much notice of them). I note that "the worst" is accepted positioned either before or after the word Czech (I tried it both ways), and indeed it will be understood to mean almost exactly the same either way. However, in my opinion the "correct" answer should be to put it after the word "Czech" as this makes it quite clear to the student that it is adverbial.
You're right that placing "the worst" after "Czech" would make it more similar to the Czech version because it would be an adverb in both languages. But I believe the adjectival word order here is more natural in English and so did the creator of this exercise when he chose the English translation.
The "sloppiness" comes from English being an analytic language (relying on syntax and auxiliary words). Inflected language in general make distinctions between individual classes of words more strongly. For example, in English you take "potato" and "salad" and combine them into "potato salad" not worrying about word categories at all. In Czech, both "brambora" and "salát" are nouns, and we first need to turn one of them into an adjective to be able to combine them -> "bramborový salát".