Each of us constructs our own unique version of the world in our minds, but we don’t necessarily notice everything, at least consciously. The things we see, hear, touch, taste, or smell evoke memories, associations and various trains of thought.
When you learn a foreign language, you are effectively creating a new version of the world in your mind. Everything will have a different name, though it takes a long time to learn all those names and there are likely to be gaps in your vocabulary. One way to overcome this problem is to learn how to describe the things for which you don’t know the words. Another way is to focus on particular subjects and to learn as many of the relevant words as you can. To test your new vocabulary, you could try explaining the subject to someone else and/or writing about it.