Don Quijote de la Mancha written by Miguel de Cervantes Saavedra is arguably the most influential book in the development of Modern Spanish. I think he has had more influence on Spanish than Shakespeare even managed to do in English, and that is really fearful. (I'm linking wikipedia because this is common knowledge in the Spanish-speaking world :)

One that comes to mind is the Russian poet, Alexandr Pushkin. He brought many words from other languages into his literature, essentially converting them to the Russian alphabet. So many words in Russian are derived from French, English, etc., and he began that trend. For instance tennis in Russian is pronounced "tyenis," (теннис) journal is "journal" (журнал).

Also, I found this in the Linguistics subreddit, might be helpful.

For Italian, you definitively want to look at Dante and specifically at the Divine Comedy as the book that defined Italian as a literary language and "created" the Italian language.

The influence of Dante on the language is enormous, and it's creation based mostly on Tuscan dialects has set the preeminence of Tuscany as the cradle of Italian for centuries to come.

Nobody even comes close in importance from a linguistic standpoint for centuries.

Later on, Alessandro Manzoni's I Promessi Sposi is considered the major opus that consolidated modern Italian at the time of the Risorgimento.

Absolutely--one of the features of most written languages is that once real literature starts to pop up writing influences the structure and vocabulary of language much faster than many other influences. The list of authors who have significantly influenced their languages, either in the spoken or literary forms, is enormous, so I'll pick out just one.

Cicero is generally considered to be the pinnacle of Latin prose, not just for his speeches but also for his essays. While both his speeches and essays were incredibly influential, it's really for different reasons. Cicero's speeches introduced entirely new methods of oratory to Rome, but unfortunately for our purposes most of those influences remained restricted to the somewhat enclosed literary circle of oratory. His essays, though, are unbelievably influential. It's quite correct to say that Cicero defines Classical Latin as we know it, as opposed to the Archaic Latin that predates him. There were a number of authors around the same time who were doing similar things, but Cicero predates most of them by a little bit and the sheer volume of innovations in the field of literary prose that can be ascribed to him is quite staggering. In fact, Cicero created literary prose as a form, since before prose was usually restricted to oratory and histories, an most Latin literature was written in verse. Cicero is also responsible from the change from the Archaic spelling of Latin to the Classical spelling that we're taught in school. In particular, declensional endings in Latin were, by the 1st Century, B.C., pronounced differently than they had been a hundred years earlier, but they were still being spelled the same way. Cicero did away with that, and at the same time did away with a lot of conventions of Latin grammar and style which were either extremely archaic and clumsy or were lousy attempts to copy Greek forms. Cicero's Latin is very much Roman and, unlike most earlier authors, it's not afraid to show just how Roman it is. The complex system of subordination found in good Roman prose of the Golden and Silver Latin periods is pretty much completely copying Cicero, who broke away from the style of most earlier Latin prose, which was highly utilitarian and compared to more literary languages like Greek very clumsy and awkward. In many instances it's difficult to tell what Cicero innovated and what was already being done before him (since we have very little Archaic Latin prose), but the influence of his prose on the language was widely recognized by Romans and later scholars. Even Caesar, who pretty openly thought that although Cicero was a valuable ally he was also an insufferable blowhard, noted that Cicero's achievements for the Romans were greater than his own, because he had only expanded Rome's physical power, whereas Cicero had expanded her spirit. And Cicero's influence goes far beyond just the Classical Period. When his letters were rediscovered by Petrarch at the beginning of the Renaissance his influence on Italian literature was incredible--Erasmus makes fun of Cicero's Renaissance fanboys for saying that unless a Latin word is in Cicero it shouldn't be used and isn't real Latin. In fact, the entire tradition of European literary letter writing is entirely because of the rediscovery of Cicero's letters.

For Portuguese, it is Luís de Camões, no question about it. He is explicitly credited as creating the Portuguese language as a distinct entity.

And he has an interesting tale (of questionable veracity) to go with it. At one point he was working on his epic, Os Lusíadas and his ship went down. He had a chance to save his manuscript or his lover. He chose the poem and the Portuguese language was saved. And Dinamene, the unfortunate lover, has been celebrated in art ever since.

/u/PoliceMachines has already mentioned Cervantes, which is what I came here to talk about, but since I'm here I'll bring up Molière.

Molière (Jean-Baptiste Poquelin) was a 17th century French comedy playwright. He was massively popular, and I absolutely love his work. He, as European playwrights often did at the time, used stock names for characters. For instance, his Cléantes are always good-hearted youngish men, Elises are always beautiful, virtuous young women, etc etc.

His characters were so well-draw that their names became the words for their attributes. Harpagon - the name of the miserly old man in L'avare - now means miser. Tartuffe - the name of the duplicitous, falsely pious trickster in Tartuffe ou L'imposteur - now means a hypocrite, especially one displaying false piety.

In one line of Les Fourberies de Scapin, a character ask what he'll do in that gallery to convey how cumbersome and exhausting the affair is, and now une galère means a cumbersome, pointless, annoying affair in French.

A scene in Le Bourgeois Gentilhomme involving the rephrasing of a line of poetry has created the phrase Belle marquise... which one says when you want to say that two phrases or arguments amount to exactly the same thing.

I know you asked about non-English languages but here are some other important influencers of English. In terms of coinages, where you rightly mention Shakespeare, John Milton is responsible for a huge amount of familiar vocabulary. Particularly negation/inversions, such as 'counterattack'. 'Darkness Visible' on Christ's College, Cambridge's website has lots of information on this.

Additionally, early compilers of dictionaries have massively influenced spelling and to some extent, syntax - from the famous example of Samuel Johnson to the politically-motivated 'Americanisation' of English spelling laid out by Webster. See Richard Bailey's 'Speaking American' for a detailed recent account of the emergence of separate American dialects.

The appropriate usage of a language in new contexts is another issue readily tackled by authors. For example, the acceptance of English in liturgy and religious ceremony. Poets such as John Donne and George Herbert helped the vernacular's slow acceptance as a language worthy of devotional use.

There are many ways that a language can be influenced beyond the addition/invention of new words. John Marston for example imported many Italian words into English usage, as well as freely making up many new terms. Of all the words used in his play Antonio and Mellida, 3% are recorded for the first time. His taste for idiosyncratic vocabulary was readily satirised by his peers - most notably, he's parodied in Ben Jonson's Poetaster, literally vomiting out new words. Being inventive with vocabulary hasn't always attracted praise!

Francois Rabelais, author of the inestimable Gargantua and Pantagruel, introduced manifold words and phrases in the French language. Many of them were loan-words from Ancient Greek and Latin but he coined numerous French-language idioms as well. If you are interested in this, Mikhail Bakhtin's Rabelais and His World is an incredible work of literary theory regarding Rabelais' use and influence of the French language.

Cicero's prose is the gold standard of Latin rhetoric. Numerous ubiquitous neologisms, such as humanitas, are attributed to him.

Goethe and perhaps to a lesser degree Schiller were hugly influential in the german language. Our version of "kiss my ass" (leck mich am Arsch) for example stems from Goethes "Goetz von Berlichingen". A very well known example for Schiller is "langer Rede kurzer Sinn" (the long and the short of it) in 'Wallenstein'. We use quite a few Shakespearean quotes in everyday German as well: Everybody knows "Alter schützt for Torheit nicht" which is based on 'Though age from folly could not give me freedom, It does from childishness' and 'Sein oder Nichtsein" (to be or not to be) might be the best know quote for which people can name the author and the play.

However, the most influential "author" was Luther, whose Bible set 'Meißner-Deutsch' as the standard for modern German.