If you're looking for a raw percentage of all existing languages, maybe you can just throw out all the major languages. Out of the thousands of languages in the world, the majority are spoken by one tribe and only internally to that tribe, and most of them may have clear answers to this question.
But assuming that isn't interesting to you, I think most of the major languages have at least one arguable "hello/goodbye" word—those that didn't invent one have usually borrowed or calqued at least one of "hey/hej", "ahoy", "ciao", "shlamah/shalom/salaam", "servus", "salveo", "good day/g'day/goddag", "aloha", etc., and use them both ways even when the source language didn't. But it's really impossible to decide which way to count each language without making a bunch of arbitrary choices. And, after doing so, the count just tells you about the arbitrary choices you made, not anything interesting about languages.
First, is "hello" really the primary English word for "hello"?
I think "hi" is used a lot more. And "hello" is certainly not etymologically or historically primary.1 The only reason to think of it as "primary" is that it tends to be the first greeting L2 learners are taught—but that's because it's the most "safe" word to use across formality and other situational factors, not because it's the word most people usually use.
And many other languages have the same issue. For example, look up "hello" in an English-Danish dictionary and it'll say "hallo", but you mainly only hear that word as a phone greeting. I think most commonly, people use "hej" in informal situations, and "god dag" in formal situations, and they just don't have a neutral word for all situations.
Dialects are also a huge issue. Probably "g'day" is primary in Australian English, as opposed to "hi" in American English. So, which of those is "primary in English"?
And that's not peculiar to English. Greeting words seem to vary between dialects (and sociolects) more than almost anything else.
It's also not even entirely clear which words can mean both "hello" and "goodbye". For English "hello" or Hawaiian "aloha", it's pretty obvious. But what about Australian English "g'day", Danish "god dag", or Arabic "Salaam"? They're all words that can mean "goodbye", but are used more often for "hello", and, out of context, imply "hello". Are they "hello/goodbye" words or not? What rule would you use?
Also, some languages distinguish between different kinds of greetings—the one you use when being introduced to someone can't be used when you run into an acquaintance, etc. So, are those not even "hello" words, or do you just have to match some of the uses of "hello" and "goodbye" to count, or some specific use, or…?
And of course these complications all interact with each other. In some Upper German dialects, "tschau" is very common as a greeting, but rare as a farewell; in eastern dialects of SHG, it's only a farewell. And in Central European languages, I can never remember from town to town whether "servus" is just "hello" or both, whether it's highly informal or stuffy for anyone under 70, etc.
And I don't think this is even just a modern problem. For example, Hebrew and Arabic both apparently borrowed from Aramaic the idea of using their "shlamah" ("peace") cognates as a greeting, but Hebrew ended up using it freely as "hello" and "goodbye" and Arabic primarily just as "hello". And of course "shalom" and "salaam" didn't enter an empty niche in the languages, they had to compete with other greetings. And things were different from dialect to dialect. Just like today.
1. It was a coinage for various specialized greetings in the 19th century and only became a generic greeting word later—I'm guessing after the telephone use took off.