Definite article before "Español".

Definite article before "Español".
 

When should 'el' precede 'Español' and when is it omitted - and do the same rules apply to the noun "Castellano"? And when should the initial letter be capitalised?

The word 'español' can be either a noun or an adjective.

An adjective never takes an article, and it's not capitalized in most cases.

When 'español is used as a noun, we need to determine the role it plays in a phrase.
-If its role is subject, an article must accompany it.
-If it's an object (direct, indirect, prepositional), an article may accompany it. Whether it has an article or not is determined by what meaning is being conveyed. If it means a person from Spain, it takes an article. If it means the language, it doesn't always need an article. 'Curso/clase de español' (Spanish course/class) doesn't contain a definite article, for instance. But it's required here: 'Traduce al español'
Perhaps a native speaker can include other examples and provide the reasons for including or dropping the article.

The initial letter of 'español is capitalized only when it begins a sentence or when the word stands alone.

The same rules apply to 'castellano, since it is also a noun or an adjective, and names a person or a language.