Like the Smurfs I am Belgian, French-speaking native and also a comic fan since my childhood. I grew reading the smurfs, in their original version. I emphasis on this because it has an importance in this debate. The answer from Nox is very interesting, but not accurate enough when it comes to the Smurf language etymology and origin, probably because he is an English-native and not a French-native, and that you need to read french to find the real explanations about this. The answer from MSilvert completes it very well and establishes the truth, but not deeply enough in my opinion.
First of all, the original name of a Smurf, is a Schtroumpf.
This is not easily pronounceable in English so they choose Smurf instead for the english versions (there are many translations for many languages). To stick in line with the French word idea, they needed a word that just did not meant anything (at that time) and that sounded well when it came to replace existing nouns or verbs with the "Schtroumpf" word and it’s derivations . Smurf was a good candidate.
There’s no doubt about the fact that the author of the Smurfs, Pierre "Peyo" Culliford, did not invent the Smurf language from something he already knew. It is also very important to understand that the Smurf language was invented before the characters themselves, and this was just a pure random moment of joking between 2 good friends.
This happened during the golden era of Belgian comics, in 1958, the friends in question were Peyo and Franquin (Franquin is even more famous than Peyo, he created Spirou and many more comics). They were together at a dinner and Peyo wanted to ask the salt shalker to Franquin, but could not find the proper word to name it… Then you can rely on the translation of that moment from MSilvert, as it is perfectly correct (except that it should be Schtroumpf instead of Smurf of course)
I asked Franquin to give me something without remembering the name :
"Give me... the smurf". I had use this word just like I would have
used "Thing" He answered "Here is the smurf, when you are done
smurfing, you'll smurf it back" We had fun smurfing for a few days. We
spend time translating Racine or La Fontaine in smurf. Also the
popular songs from back then. Which gave surprising and hilarious
So basically in a few golden seconds, Peyo invented the word, and Franquin invented the language from it. That’s it. There’s no other answer.
As an example of what they did during the days after the dinner, here’s the first sentence of the tale “The crow and the fox” from Jean de La Fontaine (original and translation here)
Maître Corbeau, sur un arbre perché, Tenait en son bec un fromage.
Maître Schtroumpf sur un arbre schtroumpfé tenait dans son schtroumpf
This is only months after it happenned that Peyo created the Smurfs, and named them after the word he invented that day and made them talk the smurf language, because him and his friends did not really stopped to joke with this during all those months.