score:12
The thing is that at the time in question, France was actually quite diverse (and yet sufficiently unified on a political level to become a rather successful democratic nation-state rather than crumble like Austria-Hungary). And as you correctly surmised, Alsace-Moselle (the German “Elsaß-Lothringen”) loomed large in French minds but not quite for the reasons you suspected.
The problem is not so much that France had no time to focus on Switzerland or Belgium but rather that it could not define itself based on some narrow ethnolinguistic features, precisely because Alsace was obviously strongly influenced by German culture. In France, it was (and to some extent still is) commonplace to contrast the “German” (ethnolinguistic) vision of what a nation is with the French vision exemplified in particular by Ernest Renan, who developed a whole theory of the nation designed to support claims that peripheral regions (and especially Alsace-Moselle) really are French.
France was, therefore, first, the area under the influence of the French kings and later a nation that sees itself as being based on the (actual or theoretical) consent of its citizens. At times (e.g. during the revolutionary wars or, in a twisted way, during colonization) there was even some temptation to consider that France could grow to encompass large parts of Europe or the world to bring progress to everyone (colonization was actually very ugly but, in France, its predatory nature was not explicitly acknowledged in the way it could be elsewhere; even the unification of mainland France itself was not always a smooth process).
The French language was therefore not a given, a way to sort people into French and non-French, but something coming from the top as a way to realize this nation. At the end of the 19th century, there are millions of French citizens (to say nothing of the colonies) who do not speak French or, at the very least, do not speak it as their first language (in Brittany, the Basque country, etc.).
It's interesting to note that during one or two centuries, the French language was extremely prestigious. It was the language of many European courts and thinkers actually debated whether it was intrinsically more suited to sophisticated thought than other languages. A bit later, in 1794, Henri Grégoire famously advocated the “eradication” of local languages and patois (the dialects many French people actually spoke) to spread the French language, linked in his thinking with progress and democracy. The common thread and relevance of all this is that the French language is presented not so much as something distinctly French as it is something France gifted to the world (but that everyone might want to speak).
In this context, it makes sense to claim territory that (in some way) belonged to France in the past rather than focus on languages and unifying French speakers is not a priority (or could even backfire badly).
Upvote:-2
First off all, these movements exist in France, too, just see how France ironed and blended smaller regions like Alsac or Occitania to a more uniform French nation. Such kind of national movement was already present during e.g. the French revolution, where the properly cohesive force was the national identity.
What you forget, your examples about national states where state boundaries were significantly different from national boundaries and/or nations with no independence/own states (unlike in case of France). Slavs were never even close to have a unified, big empire, not small part because slavs do not speak the same language, and also national identity existed even from the start: just take a look at Czechoslovakia, Yugoslavia... Panslav movements (much like Italy or Germany) was just a way to justify independence from the feudal structure at that time, justify an opposition or to justify the Russian influence. Not accident that Poland was generally less enthusiastic about panslav ideology.
Italy or Germany was also just a concept, to gain political influence over an extended region when present feudal structures were unable to hold them. It is not only about languages. Just as France did not melt in every French speaking region, Germany did not include Austria, german-speaking Switzerland, Luxemburg, Netherlands (local German dialects are rather close to Dutch), in spite many of these regions are traditionally part of Holy Empire or the Habsburg Empire and culturally very close to Germany.
Let me also point out that XIXth century Europe, especially Central and Eastern Europe, were not ethnically that clear as most people retrospectively think. Most ethnics were mixed all over the place: the previous centuries filling the villages with settlers due to larger population movements following the wars and plagues, and as well as towns. Most town population e.g around the Austria-Hungarian Monarchy were bi or trilingual.
Upvote:2
Actually, there was a mild form of French "Irredentism" that manifested itself in the Austro-Sardinian war of 1859, when France demanded the return of Savoy and Nice in exchange for aiding Sardinia in a war against Austria.
When France overran and occupied Switzerland, Belgium and other nations under Napoleon, she illegitimately claimed control of more than French-speaking territories. After the defeat of Napoleon, the more "legititmate" types of irrendentists (if any) pretty much kept quiet.
After the Congress of Vienna in 1815, when France's borders were "rolled back" to pre-Revolutionary times, France became a "model citizen". France was later admitted to the "Concert of Europe" which solidified her status as a "Great" (European) power, but needed to be on her best behavior to maintain this position. After that, the only French territorial claims (in Europe) besides Savoy and Nice that were considered "legitimate" were those in Alsace and Lorraine, against Germany, the new, rising "aggressive" power, which took those provinces away from France in 1871.