Wrongfully accused in Leroy Merlin

"We were queuing to pay at the till and my husband said he was going to go to the car with the samples"

That was daft, 

"The police gave us a court date in February and said my husband has come back to get fined by a judge. They made my husband sign something too"  I think you should get a consultation with an English-speaking lawyer. Who hopefully will set thing right with the store and the police, if you don't want him to go to court. 

As already said it was silly taking the samples to the car without the things you had purchased, no real reason for it. But more importantly why didn't he or you get a member of Leroy Merlins staff to agree they were samples and that he was allowed to take them, after all the speak English?

I would go back to the store with the samples and with the jobs worth security get it agreed in writing that it was samples. Let the manager or similar know about court and unless agreed beforehand you will be looking for your costs. 

Again, get it in writing. Good luck!

As said not the wisest move to walk out the store with unstated number of large samples. Hindsight wonderful would have been so natural to ask when paying for your other goods, to ask "is it OK to take these"

Take this seriously, get legal advice. Surely you don't believe this "its only a fine", if it's a fine for theft it's a fine for a crime

"As already said it was silly taking the samples"

Indeed, and not labelled 'free samples'. (Muestras gratis) Store samples. 

Were the sames pre cut or did you cut them off yourself, which may be the problem. 

If you think of your self as the vendor then that's stock they can not sell. 

I worked in decor retail and this was a huge problem but in the UK we didn't call the police. 

We had small samples at each roll for customer use 

In the present case, it will be difficult to share your view of the matter.
Basically, it is annoying for every trader that parcels are opened and parts of the goods are taken as "samples". Opened parcels are difficult to sell. If one absolutely wants to take samples, this can only be done with the involvement of a person from the sales staff. Imagine if every customer were to help themselves to packaged goods and do so past the checkout... Afterwards, you can only try to contact the management of L.M. Malaga in the hope that they will withdraw the complaint against you, because if the matter is effectively brought before the court, L.M. will be represented there by a lawyer.

We had signs on the wallpaper section saying 

Lovely to look at, lovely to hold. If I am open 

consider me sold. 

I can understand. In home décor stores in Ireland you can take small samples from open packs, which are displayed beside the new unopened packs, for that reason. What an awful ordeal for you both. I hope you're okay.