I heard that we cannot describe the feeling of a third person using たい, ほしい, 楽しい, and others? Is it true?
For example, the following sentences are considered incorrect.
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It is not exactly incorrect to do that, but you should try to avoid assuming other people's emotions because you can never know them for sure.
The way to get around it is to add uncertainty to the statement. Most commonly, at least in casual conversation, you use でしょう at the end.
彼はピザを食べたいでしょう - (it seems) he wants to eat pizza.
You can also use the -がる grammar for adjectives, but this often has an impersonal and distant feel to it, so it's not used as often.