Would Soviet assistance alone be sufficient for France, though? After all, in World War I, France and Russia couldn't defeat Germany without British and American help!
The USSR of 1938 is very different from the Romanov dynasty ruled Russia of 1914 to start with. It isn't nearly as formidable as the Red Army of 1942 despite massive Soviet losses of men, material and territory, true. That's mainly down to inexperience and a failure to weed out the politically favored deadwood-OTOH in 1938 the Army Purge has not happened yet either. Meanwhile the Wehrmacht of 1938 is a very dubious comparison to the Second Reich armies of 1918 too, again despite the devastation and privation of basic supplies the Germans faced by that late date in the Great War. There is speculation that the Czechoslovak forces of OTL, before surrendering so many assets to Hitler and deposing their government in favor of a leader handpicked by Hitler, could, if they had resolved to fight instead of submit to Munich, have made a pretty brave showing against the German forces of 1938. If all the Soviets send are token forces, they won't make much difference, and I suppose the case might be argued that man for man, even the Werhmacht of '38 might have had a superior loss ratio and higher effectiveness--man for man. But it was not yet nearly as expanded as it would be just a year later when Poland was attacked OTL, whereas the Red Army had behind a tremendous current enlistment, a generation--at least half a generation anyway--of former conscripts suitable as reserves, all of whom had been through peacetime service. If Stalin were resolved to stop Hitler from taking Czechoslovakia I think the only way it would go badly for him would be if the Germans could break through so quickly that the major strategic assets of Bohemia are lost before substantial Soviet forces could get to the field.
The political equation is something else. I think maybe if Czechoslovakia had shared a border with the USSR for a long time, it is possible this would considerably chill political support in Czechoslovakia for a Soviet alliance. If this were not the case, realistically, whatever the prevailing feeling is in Czechoslovakia, admitting enough Soviet troops and other forces to assure defeat of the German invasion would put the country at severe risk of a Communist takeover with even less pretense than OTL of political legitimacy; the presence of the Soviet forces could shield a simple Communist coup.
Would Stalin wish to rush to Czechoslovakia's defense if the way were open, geographically and politically? It is my general belief that he was the Great Procrastinator, motivated by both Bolshevik ideology and personal dictatorial reasons to form a vast, mighty army reasonably well modernized and on paper able to take on any power, great or small. But generally he would never be the first to initiate a war in part because of his fear of empowering some successful marshal to take him down in a coup--he had other reasons to hesitate to strike the first blow too, boiling down to the political value of being seen as the victim versus as an aggressor. That did not stop him from doing things that outraged the morals of the Western liberal powers when it suited him, when he was well covered from the immediate consequences of their outrage anyway.
But Hitler attacking a defiant Czechoslovakia is a different case. Were he to honor his own offers to help, and the Czechoslovak government to take the offer, no Western power would see him as an aggressor. A cold blooded opportunist perhaps, especially if the Western powers do nothing effectual which leaves Stalin effective master in Czechoslovakia. I think he would calculate that the Red Army could probably get to the front soon enough for numbers, combined with Czechoslovak arms, defensive geography, and resolve to defend themselves, to prevail and move the fight, if the Germans do not capitulate first, onto German soil. Meanwhile the Germans do not threaten the Soviet Union directly anywhere; they would have to persuade some allies to strike on their behalf, and those in a position to do so--Poland primarily, and Romania, and to a much lesser degree Turkey, and of course Britain via Indian territory--would hardly be motivated to, unless they feared the Red Beast was unleashed and would inevitably attack everywhere. But by coming in as Czechoslovakia's ally, once the basic business of holding or regaining prewar borders was accomplished, it is at Stalin's discretion, with the permission of the Czechs of course, whether to double down and start conquering the Reich, or seek a cease fire with terms for Germany. Terms that the consensus of the liberal powers would probably find reasonable under the circumstances would probably include removing the Nazis from power, outlawing Hitler, and breaking the Reich back up into a separated Austria and Germany. Indeed he might seek even harsher terms, such as disbanding Germany into smaller states. A new Versailles Treaty again placing Germany, including Austria, broken up or not, under indemnity and limitations might also seem entirely in order. Or he could either refuse to hear terms, arguing before Western public opinion that no German treaty could be trusted, that removing the Nazis might be accomplished on paper but only occupation and forced reforms could free Germany of her barbaric dictatorship in practice, or put out terms so harsh the Germans do not surrender. Eventually Germany would be cut in two, with westward Czecho-Soviet salients out of Bohemia reaching the French border and severing Austria; Austria herself overrun and probably turned into some sort of People's Republic; the slog northward would be harder but inexorable I think and when the dust settles, both Germany and Czechoslovakia are either Soviet puppets or incorporated as SSRs outright, perhaps five or six of them--Slovakian, Czech, Austrian, Bavarian, Baden-Wuerttemberg, Saxony, Hesse, and a few others--maybe Hannover comes back on the map, probably Prussia is forthrightly dismembered much as OTL. This would leave Poland and the Baltic states in horrible isolation of course, only the Baltic their lifeline.
All this if the USSR and Czechoslovakia fight alone. It could be some other powers get drawn in even if the Entente powers sit it out--perhaps early on Hungary and maybe even Poland can be brought in on the German side which would surely distract Soviet strategy based on liberating and moving out of Czechoslovakia, but in the longer run would merely engage and commit the USSR for an all in fight and result in Red rule east of the Rhine. The best case for Czechoslovakia is that seeing the Russians move for real, the Entente powers tear up the Munich accords, or we are in an ATL where they never signed them in the first place, and France and Britain attack Hitler for breaking both the Accord and the Versailles treaty. Probably they get a resolution through the League of Nations calling for common action to move into Germany and remove its current regime. If this happens, Stalin loses the opportunity for eastern European empire, but I don't think he was counting on that; instead, the bloodletting of the Red Army will be shared by Entente powers. The best thing for the Czechs, who could still come under Soviet dictatorship anyway, would be if the French and/or British offered to move in significant forces of their own via Romania--Romania signing on to the anti-Hitler crusade would be the best thing for deterring Soviet designs on Romanian territory and independence too. If Red Army boots on the ground in Czechoslovakia are balanced by British and French ones, the liberal government can hope to stay in power. I think Stalin would be prepared to live with this, provided the Third Reich is dismantled and the Soviet Union gets a fair say in the fate of Germany. Aside from any expeditionary forces taking a roundabout path through the Black sea to get into Czechoslovakia, obviously France could attack on the German western border, while Britain interdicts all trade. League of Nation mandate can guarantee Belgium, the Netherlands, and Denmark close their borders, and Sweden persuaded, if not right away than as the pendulum swings against Hitler, to terminate their trade with Germany too.
The main reason I am confident that Soviet aid to Czechoslovakia is sufficient to crush the Reich in time is that Hitler has not had time to develop the forces he intended to contend with the Great powers over Europe with yet. Soviet forces will probably prove somewhat inefficient but they have the numbers to bleed out a lot and still get steeled by painful trial and error. It may take time for the Red Army to learn how to make its numbers tell, but that only buys time for the Germans, and perhaps other nations Hitler might persuade to jump into the abyss with him, to kid themselves into thinking they will come out ahead. Eventually the Soviet steamroller will get up some momentum and once it does, the Reich and anyone allied to it is through.
Unless perhaps the Entente powers, seeing that the Red Army need not be defeated and Czechoslovakia with Soviet help can hold, would get on the bandwagon; they might thus win a position to share setting terms with Stalin, and thus have leverage to persuade Stalin to stop when they say so. That just means the pile on to break the Reich is all the heavier of course, and the wrting on the wall all the clearer and brighter.
The Reich alone,
given time to arm, might indeed be able to check all other powers without Soviet help, and perhaps able to fight the Red Army to a draw instead of certain defeat if able to keep the fights purely bilateral.
Without time to arm, I believe even say France could do it with help from Czechoslovakia, and an expeditionary force could make its way to Czech frontiers via Romania. The British would be hard pressed to win if fighting alone--but with a roundabout path for expeditionary forces through Romania, even British forces might suffice to tip the balance.