Learn Russian: Они транслируют новости каждый вечер.

Транслировать (to broadcast) is a biaspectual verb, meaning it can be either perfective or imperfective depending on the context. Here, the phrase каждый вечер (every evening) tells us the action is repeated and habitual. Because of this trigger, транслируют is resolved as an imperfective verb.

Since biaspectual verbs use the same conjugated forms for both present and future, how do we know this isn't the future perfective 'will broadcast'?

You have to look at the context clues. Biaspectual verbs like транслировать do indeed use the same conjugations for the present (imperfective) and simple future (perfective). However, каждый вечер points to an ongoing routine, firmly placing this in the present tense. If the sentence instead used a word like завтра (tomorrow), it would be understood as the future perfective.

In English, 'news' is a singular, uncountable noun. Is it the same in Russian?

No, Russian treats новости as a regular plural noun. The singular form is новость, which means 'a piece of news.' In this sentence, новости is the direct object in the accusative plural. If it were the subject, it would require plural verb agreements, unlike English (e.g., 'The news is good' vs. Новости хорошие).

Why is каждый вечер in the accusative case here?