The classes (B4XPages pages) are located outside the folders relating to the individual "languages" and are linked. You can verify this by looking at the icon next to the file name, in the "Modules" tab; it has 2 dots between round brackets.
When you create a new page, you are asked: "Add module to parent folder?". If you flag that checkbox, the file will be saved in the "common area", i.e. the project root folder, instead of that of the language in use at that moment.