Hm. Wittgenstein requires that the meaning be "indicative". In English the indicative mood is used to express statements of fact, or which are very probable. They don't necessarily have to be true or probable, of course, but they express beliefs of that nature. You say "I believe X" when you assign a probability of at least 0.8 to X; 0.8 is probable, but not very probable. Would you state baldly "Barcelona will not win the Champions League", given your probabilities? I doubt it. When you say instead "I believe Barcelona will not win the Champions League", you could equally say "Barcelona will probably not win the Champions League." But this isn't in the indicative mood, but rather in something called the potential/tentative mood, which has no special form in English, but does in some other languages, e.g. daro in Japanese (which has quite a complex system for expressing probability). It's better to just say your degree of belief as a numeric probability.