Btw. Does anybody knows some alternative to learn to speak/understand Japanese ?
However, learning hiragana is, I think, absolutely necessary. Why? They are the building blocks of how Japanese actually sounds.
I have met hundreds of non-Japanese people who have spent time living in Japan and trying to learn the language.
Many can barely read kanji, but are fine with everyday listening/speaking.
But those who refused to learn hiragana? Their pronunciation is terribly difficult to understand, and their listening skills suffer, too.
My takeaway: learn Hiragana first.
Personally I think there might be some correlation vs causation on hiragana vs pronunciation, many people with poor pronunciation just have a mostly-English speaking social circle, and as such likely also don't feel a need to study hiragana. I feel just listening to Japanese can help get your brain used to the sounds and on the flip side my first recommendation to Japanese to get better at English conversation is to watch lots of US TV, hearing words over and over from many voices eventually adds up to a nice base for sounds. Still need to study for vocab though.