A note on languages and me: I haven't grown up bilingual, so my experience of learning a new language is entirely different from a kid who had to learn that there's a difference between English and German. (I've had an interesting discussion with a dad that raised bilingual sons. He said that his kids had to retrospectively learn that "table" and "Tisch" belong to different languages, although they mean the same thing. It took them a long time -- well into High School, in fact -- to separate the two languages, and one school even threatened to expel them if they didn't clean up the language mess they frequently produced, mixing up two languages in a way that hardly anybody else could understand.) Nevertheless, English has become an integral part of my life, and some time ago I've started to learn the meaning of an English word, and not its German "translation" -- yet, there's German left in my English, especially in the grammar and in the way I use certain words.
With this experiences in mind -- and the fact that I've been working on a project that heavily relies on a pidgin language that nobody speaks anymore --, here's a few thoughts on your questions:
There's a difference between the language that your characters use and the language that the story is told in. You can have a story about a Japanese child that is growing up in post-WWII Hiroshima that is entirely written in English. The reader knows that the child is Japanese, and that it speaks Japanese and thinks Japanese, but s/he doesn't expect a Japanese text. When you chose your "manuscript language", stick to it. The trouble with every language that deviates from your manuscript language is that there will be reader who doesn't understand it. If the part that is told in the non-manuscript language is important, you will have to provide a translation. How will you do this? Footnotes? A glossary at the end of the book? Translations in brackets? Each of this ideas is valid, and each greatly diminishes the readability of your text.
Why is it important that your characters speak different languages? From the top of my head, I can think of only one example that justifies a character speaking a foreign language, and that's when the characters are supposed to not understand each other. (That's actually the only reason the pidgin language has to show up in my story at some point: It's a code that the social class of my characters has developed to separate themselves from the rest of society. It is their explicit goal to not be understood, a fact that also serves to define them as a social group.) Consider this:
It was Sally's first night as an exchange student in Hamburg. Daniela had picked her up from the airport and driven her home, now she had gone out to a Cello lesson and Sally stayed behind in her parent's apartment, tired out by the long flight but excited to be in Germany. When Sally was unpacking her suitcase, there was a gentle knock on a door, and a silver-haired woman of about 50 years shuffled into the room. She smiled nervously at Sally.
"Also ...", she cleared her throat and proffered a hand to Sally, "hallo, Sally. Ich bin Andrea, Danielas Mama. Daniela hat schon erzählt, dass du kein Deutsch sprichst. Das könnte ein bisschen schwierig werden, weil ich leider kein Englisch verstehe, aber wir kriegen das schon hin, eh?"
Sally stared at the hand of the woman. She hand't understood a word of what she had just said, but was determined to make a good first impression. She shook the hand of the woman, concentrated and said the one sentence her brother had taught here before she had left for Hamburg:
"Hallo. Es freut mir, Sie kennen lernen."
That was supposed to mean: I'm happy to make your acquaintance. From the way the other woman smiled -- bewildered and a bit touched by Sally's heroic efforts --, Sally suspected that she hadn't got it quite right.
The obvious problem is that this scene won't work for a part of your readership. The part that speaks German understands what Sally's Mom is saying to Sally and can't really share Sally's confusion; the part that doesn't speak German doesn't understand why Sally's well-intended German sentence doesn't work out.
A different version of this scene's dialogue could look like this:
The woman cleared her throat and proffered a hand to Sally, rapidly saying something in a language that Sally suspected to be German. She didn't understand a word.
The woman still looked at Sally. Her facial expression said: I know this won't be easy for either of us, but we will find a way, won't we?
That's probably what I would try to do: Tell you reader that a certain language is used, but don't actually use it.
As I tried to demonstrate above, each foreign language that you use in your text will introduce some level of confusion to one part of readership or the other. That's why I would try to avoid this entirely. However, you're free to state which language your characters use with phrases like "(...), he said in Hindi" or "To make sure only her best friend understood her, she continued in Amharic: (...)", if the fact that your characters speak different languages is important to your story.
Also keep in mind your narrators. If your narrator doesn't speak Amharic, s/he can suspect that the other character speaks Amharic, but s/he can't know for sure. This is different for an omniscient narrator.