"They are very strong substances."
Translation:Son sustancias muy fuertes.
is 'fuertes' one of those adjectives that doesn't need to agree with the noun it modifies?
"Esas son sustancias muy fuertes" is incorrect??? NO! If you are going to forgo the article, then an article is not required in the translation. If you want the user to include the article, then INCLUDE THE ARTICLE.
FFS PLEASE REMAIN CONSISTENT!