Outch.... "Rentrez chez, soldats français" doesn't mean nothing
I would have translated "go home french soldier" by "casse-toi sale français !" (Get lost you bloody french !"). You could also use the word "frank" which is used by breton nationalists like in this song.
Rather than "casse toi, sale francais" (that would be translated as "Go to hell, dirty frog", and far from the "US Go home" he tries to rend) it would be correct to put "Rentres chez toi, soldat français".
But if I understand it well, and that both states are french-speaking and originated, making insults as "franchimand" (les bretons n'ont pas vraiment le monopole des appellations injurieuses) seems a bit weird like spanish nationalist insulting republican for being spanish.
So the poster makes little sense to me.
Well in terms of translation, using overall "Go Home French Soldiers" was translated as "Allez-vous chez le soldats francaises" but that didn't fit on the poster haha. I saw and was going to use Rentres chez toi, soldat français but that again was hard to fit and have without different font sizes, which I didn't want to do. I asked someone studying French if simply "Rentez chez soldats français" made sense and they consented so I went with that.
In terms of the meaning behind the poster, the Republic of Gaul lasted a few decades and in that time, thanks to a loathing of the French Syndicate, tried to develop their own identity. I almost thought about trying to find a way to say "Go Home French Soldiers" in an old Gaulish language but that was rather hard to find. So for the Spanish Republic example, it would be Catalonians insulting the Spanish in Spanish. Looking at the source of the poster (the IRA-centric Go Home British Soldiers) being an Irish group using English would be the same notion as the Gauls using French for the poster as most people could only read it that way.