A court (assuming it is a regular court) can only adjudicate within its jurisdiction. So a lawsuit filed in a court would have to refer to laws within the courts jurisdiction. No court would use the laws of another jurisdiction since it would basically be unqualified to make a judgement.
If one party in country A wants to sue a party in country B, they have to determine which laws are in force in which country and where the faulty action occurred. There is no point suing in country A for a law only in effect in country B and vice versa. And very often if the alleged faulty action is committed in country A, the courts in country B will refuse the case (this is not ALWAYS true but nearly always).
There is this content called “jurisdiction” this means that court normally only can hear lawsuits about thing that happen in the place they are in and apply the laws of that place and the time where things happened.
For example if you want to sue someone in a French court, the thing you are suing them for should ideally have happened in France and French law will be applied.
When something happens across borders things get more complicated. Business who deal with each other internationally ensure that they make clear in their contracts where disputes get handled and treaties between countries exist to remove some uncertainty about which laws apply when.
Sometimes you get weird cases where a court ends up applying the laws of some other place and time when rather than the current ones where they are, because those are the ones that applied to the things as they happened.
In some cases in criminal trials some countries like to expand their jurisdiction beyond their own borders to apply their laws outside their own territory, but that is controversial.
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