When it comes to worldbuilding, especially fantasy worldbuilding, constructing a language is the sort of thing you sometimes feel you ought to do, especially since Tolkien set the standard by not only building several languages, but their historical evolution.
That's hardcore linguist stuff, and you shouldn't go there unless you've got love for it.
And let's face it, the bulk of the constructed language stuff only exists for small percentage of purists in your audience.
But, let's say you wanted to have that hint of the larger language-- just that hint of verisimilitude, so when you drop an "othered" word in your manuscript it doesn't feel like you threw a handful of Alpha-Bits on your desk to get your word.
So, the easiest way to accomplish this is not to worry too much about grammar and vocabulary. If you don't need to construct complete sentences on a regular basis in your manuscript, there's no need for it. But what you can do, without too much work, is figure out what the building blocks of the words are, and use that to create a sense that the language has a consistent framework.
One way is to familiarize yourself with the International Phonetic Alphabet, which is a simple, codified way to express about every sound the human mouth can make. You don't have to memorize or master it (I certainly haven't), but get a sense of it.
From there, you can figure out what phonemes and morphemes can exist in the language. You don't need to come up with an alphabet, or even if the language is alphabetic (like English and Romance languages), syllabic (like Japanese) or logographic (like Chinese). You just need to know what sounds are allowed.
Now, devise a consistent way to express these allowed sounds. In a way, this is creating the alphabet, but specifically you're creating the transliteration of your language, using the symbols you can readily type. (You could just use the IPA, but that might frustrate readers.) But it's important to make it consistent. English can be a maddening language in that sense: "straight", "wait', "weight" and "late" all rhyme, but express the same vowel sound four different ways. This can also minimize your own confusion when you go to make a new word.
This will also prevent you from making typical "fantasy language" errors-- namely, throwing in accent marks or apostrophes or excessive use of the letters "æ"or "y" to make it seems like a Fantasy Language word. I'm not saying you can't use accent marks or apostrophes or the letters "æ"or "y", but if you create a clear set of transliteration rules, then they won't come off as random. They'll be a consistent feature of the words of that language.
And that will make it feel more real, without having to invent the whole thing.