French is a language from the Latin or Romance language family Although it has influences from far and wide, It originally evolved from "Vulgar Latin" which was the everyday language of the Roman Empire.
This means that French is closely related to most of the languages that the Roman Empire dominated some 2000 years ago. This includes Spanish, Italian, Portuguese, Romanian as well as several lesser known (or smaller in population) languages and dialects.
Since then, French has been influenced by a number of other languages, but it has also left its mark around the word, meaning that French has a lot of foreign languages in it, as well as there being a lot of French in many other languages. (Especially English!)
In the following, I'll try going through a few languages in order to try and find out which language is the closest to French.
French's first cousins, Italian, Portuguese and Spanish
These three are distinctly Romance languages, and looking at any of them, lets you see similarities.
French isn't mutually intelligible with any of them, though. People who speak Portuguese and Spanish are able to communicate, each one speaking their own language, and I once witnessed a discussion between an Italian and a Spanish flight attendant. But French seems a little like the odd one out in this group of languages.
But still, there are similarities that you can't deny. If a French speaker makes an effort, he or she should actually be able to get the gist of Spanish writing.
Here's an example from the Spanish version of the Universal Declaration of Human Rights:
Todos los seres humanos nacen libres e iguales en dignidad y derechos y, dotados como están de razón y conciencia, deben comportarse fraternalmente los unos con los otros
Let's compare that to the French version:
Tous les êtres humains naissent libres et égaux en dignité et en droits. Ils sont doués de raison et de conscience et doivent agir les uns envers les autres dans un esprit de fraternité.
They actually look a lot alike. I speak French, but I don't speak Spanish, but apart from a few (two or three) words in the Spanish text, I can actually understand it when reading it.
Spoken Spanish would be another story though.
I've heard people comparing French and Portuguese pronunciation. Personally, I don't think that they have a lot in common, and I don't hear a strong similarity to French when listening to Portuguese.
But perhaps there's a slight "Quebecois" vibe to it? Portuguese vowel combinations just might resemble the French dialect in Quebec, but I think the similarity between the two stops there. (Have a look at this video in Portuguese and compare it to this one in Quebecois)
Italian, like Spanish, is somewhat decipherable in the written word, although not as much as is the case with Spanish. I've heard people compare French and Italian grammar, and while that might be a thing, I don't think it really helps understanding the language in terms of listening and reading.
Haitian Creole is a language with an obvious connection to French.
The history of Haitian Creole is not a beautiful story and it deeply marked by colonialism and slavery. Today, however, Haitian Creole is a thriving language. It's got a distinct French sound to it, but there are a lot of other influences visible in the language as well, both from the European and African continent.
The strong French influence means that Haitian Creole and French are more or less mutually intelligible, but it's clearly a language of its own and not a French dialect. The term "creole" is widely used for "combination" languages. In other words, languages that mix elements of two or more languages. While this, of course, is true for Haitian Creole, it can be said about all languages, French included.
To listen to spoken Haitian Creole, have a look at the video below:
Occitan, strangely close to French
Another language that's closely related to modern day French is Occitan. Occitan is also known as "lenga d'òc". It's a language mostly spoken in Southern France, Northern Spain and to some extent, North-Western Italy. While Haitian Creole is a relatively new language, Occitan can be traced back more than 1000 years.
Although Occitan remains a minority language and is only spoken by a few hundred thousand people, it's a distinct member of the Romance language family. Spoken Occitan has some clear parallels to French, and sounds strangely familiar to someone who knows French. You can actually somewhat understand the language, and it sounds somewhat French, but the words and grammar is different.
To listen to an example of Occitan, watch this video below:
Along with Occitan, there are several other minority languages in the region that seem strangely familiar to French like Gascon and Provençal.
So which language is closer to French?
The question is difficult to answer. I could have written a much longer article without coming any closer to a solution. Some consider Quebecois a separate language from French, other say it's a dialect. If you were to consider it a separate language, there would be no doubt that it was the closest.
The big languages in the Romance family, such as Italian, Spanish and Portuguese (and to some extent Romanian) all share an important amount of vocabulary with French, and the grammar can be similar. But not to a point where French people understand the other languages merely from hearing them.
The closest languages probably remain Haitian Creole, Occitan and other smaller languages that are ether regional languages or strongly influenced by colonialism.
But what do you think? Please let me know in the comments below!