It's vanishingly rare to need a constructed language in written fiction.
Orson Scott Card sums this up in How To Write Science Fiction And Fantasy:
Invented languages are a lot more fun to make up than they are to wade through in a story.
Here's the thing: very few readers will have the patience for more then brief, occasional snippets of languages they don't understand. And for brief, occasional, you really don't need the depth and authenticity of a constructed language - almost nobody will care enough to check up on your linguistics.
There exist very effective ways to convey foreignness while keeping the individual words clear. Here's the relevant page in Card's book.. He gives two basic approaches:
- Use snippets of the foreign/fake language, and then immediately translate into English.
- Don't use any foreign language at all, or very little; automatically translate into English. Make the culture foreign rather than relying on made-up words. Card gives an example of this:
"God give me strength not to kill you for having seen my ugliness," he said to me.
I blinked once, then realized that he was speaking Samvoric and had given me the ritual greeting between equals. I hadn't heard Samvoric in a long time, but it still sounded more natural to me than Common Speech. "God forgive me for not blinding myself at once after having beheld your glory," I said.
Then we grinned and licked each other's cheeks. He tasted like sweat. On a cold day like this, that meant he'd either been drinking or working hard. Probably both.
So clearly, you can get along pretty well without a constructed language. All you really need is:
- A general, hand-wave-y sense of what the language might sound like.
- Maybe a few unique words, phrases, and concepts.
- Good writing technique.
You can get very, very far with these - and you'll generally be more reader-friendly, too.
Sometimes, though, there are good reasons to go ahead and make up a language anyway, even if it isn't strictly necessary. In my eyes, the main benefits of a constructed language (vs. faking it) are authenticity, and richness - a fully-constructed language is just more real than making stuff up.
Here are some circumstances where I'd recommend considering a constructed language:
- You care deeply about the authenticity of your setting, and see this as worth the investment.
- You see developing a constructed language as a tool to developing and fleshing out the culture which speaks the language - a worldbuilding tool. The actual language may rarely (or never) actually appear in your story, but you'll know how crucial it turned out to be that your culture has five thousands words relating to "survival" and none relating to "food."
- Your story deals with language or translation as a significant, primary element.
- You're writing for TV or film, where you can get in lots of foreign-language dialogue because it doesn't bring the action to a halt (as it does in prose).
- You have, or would like to cultivate, a fanbase interested in this culture, and you think having a language to sink their teeth into would be exciting for them.
- Inventing languages is a hobby of yours, and you enjoy doing it even if the language isn't ultimately necessary or prominent.
I'm sure there are other good reasons (and other bad ones); the important thing is that you know what you stand to gain by constructing a language (and how important it is not to lose your readers by swamping the story with it). If you understand the pros and cons, you should be able to make your decision on your own particular case.