The question is reasonably asked as to whether both singular and plural forms should go in a particular lesson; the answer is both.
For example, in the Animals French lesson you will find both the singular and plural words for:
- goat and goats.
- horse and horses
The reason to include both, is that often there are special differences between the singular and plural (such as between horse in the singular (cheval), and horse in the plural (chevaux)). Rather than have to think whether or not there is some special reason to include both forms, it's easier to just have a general rule that both forms are included.
There are many words which currently don't have the singular and plural forms, so that is why you will not see both singular and plural form in the lesson (if the words do exist in Lingopolo and they are not in the lesson, then they should be added).