Requested by:  This took me so long to write because I lost your request?? I'm super sorry about that, but here you are!

When Their First Language Isn't English: A lot of authors will get this wrong if they don't understand... well, how being bilingual is. The most common thing you'll see if the bilingual character doing something like this.

"Voy al cine en Sabado porque me gusta peliculas."

"What?"

"Oh, sorry. It's hard to switch back. I'm going to the movie on Saturday because I like movies."

That's not how it works! You don't "switch" unconsciously in full sentences. Obviously, you can talk to people in one or the other (fluency varies depending on the person), but this is a much more accurate accidental "switch"

"Do you know where my uh... the um..."

"What do you need?"

"You know, coat but for inside? Hijo de puta, la sudadera! The- the sweatshirt!"

If a bilingual person is going to struggle for a word, they're going to have to mentally translate, and that can lead to thinking it through out loud. It's not a switch back and forth, it's an always mentally translating.

Writing Translations: There's two ways to do this and both work well. You can write the actual sentence in the actual language and put the translation immediately after or at the end of the chapter.

Yo cuidado mi hermano menor. (I take care of my little brother)

Or you can make a note somewhere in the story saying something like "bold italics are the characters speaking in Spanish" and then just write whatever you want in the different language in bold italics.

I'd recommend the second one if you aren't fluent in the language and you want to write a lot in the different language, unless you have a really good, reliable translator. Because Google Translate isn't the best and it doesn't cater to cultural differences (such as formal vs. informal, tenses, etc.). However, you can still use a bit of the language in your story legitimately. You can have a character call their Grandma "Abuela" and still not write full sentences in Spanish. 

All in all, just be sure to be accurate and true to the language/culture you're going to be writing about and representing in your story.