Can you help me understand...?
Is “principalement” part of the noun phrase in this sentence? Ces écrits ont été rédigés principalement en tchèque.
No, in this sentence principalement is an adverb describing the word rédigés.
Translation: These writings were written primarily in Czech.
The ending "ment" at the end of the word is like adding "ly" to a word in English, usually making the word an adverb that is describing the action in the sentence.
For example: Evidement = Obviously
Tristement = Sadly