When do you use “the” before a language? When do you not use it?

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For "language" you mean: Spanish, English, French.. ?

When we talk about school "the" can be removed.
Usually when we refer to school subjects the article "the" is not necessary. Although when it is an object complement it is usually used.

"At high school I will study French."
"Alle superiori studierò Francese" >> more natural
"Alle superiori studierò il Francese" >> still good

You have to use the article "the" when language is the subject of the sentence.
"Japanese is difficult"
"il Giapponese è difficile"
OK, here it can be misunderstood with "the japanese guy is a tough one"

When in a movie there is a foreigner who speaks a bad Italian (or another language) usually he doesn't say articles and doesn't conjugate verbs. I think this trick is common in all films in the world, each with its own grammar.

"English is easier than French"
"L'Inglese è più facile del Francese" >> natural
"Inglese è più facile di Francese" >> As Italian this sentence sounds to me like a caveman talking. (or as a beginner studying my language, i'm sorry).

"I like ice cream"
"Mi piace il gelato" >> natural 100%
"Mi piacere il gelato" >> Fred Flintstone level 60%
"Piacere il gelato" >> Fred Flintstone level 80%
"Piacere gelato" >> Fred Flintstone level 100%