Which japanese language is mostly used in Japan?

Yes, I had trouble too understanding your question. Japanese uses three different kinds of writing. Originally had no written form and took the characters from Chinese and their written language. However, Japanese is very different from Chinese and soon they found that they needed some way to write Japanese with its many inflections. So Kana came about as an aid in writing Japanese in Chinese characters. For example "I go to the store." 私は店に行きます。　Has　Chinese characters：　　私・店・行く（the root form of this verb is written in Chinese but the flexion is in Kana.)
watashi / mise / i(ku) Iku is the common way to say go. Ikimasu is the polite way so you have the root i(kimasu) Chinese(Japanese kana) watashi wa mise ni ikimasu wa and ni are particles. In the case of wa and ni in this sentence you could call them case marking particles kakujoshi. Particles in Japanese are almost always written in kana like the endings of verbs and adjectives. Usually this is hiragana. Katakana is used more for foreign words and when you want to EMPHASIZE something you can write it in katakana. Then romaji is used to help people who cannot read Chinese or Kana, I think. Actually you could write anything in Japanese in hiragana or romaji. But, normally it is written in the combination of Chinese characters, Kanji and Kana both of them but mostly hiragana. I sometimes see English words too in Japanese sentences. I mean regular English words and no romaji (romaji is how you write Japanese in western letters -- different from English)

There is only one Japanese language, but it has three writing systems : kanji (Chinese characters adopted by Japanese) and the two kanas : hiragana and katakana. Most Japanese writing is a combination of kanji, kanji roots and hiragana suffixes.

Japanese primary schoolkids learn hiragana in their first year, before tackling kanji. A method followed by many foreign Japanese learners.

Katakana is mainly used to write foreign language words, for emphasis, to represent onomatopoeia, and to write certain Japanese language words, such as technical and scientific terms.

Romaji is mainly used to learn foreign learners the pronunciation. It has virtually no use inside Japan. Many experts give the advice not to bother with romaji, but to start immediately with hiragana.

they are both same language
romaji is just romanization of japanese, like watashi wa...you cant read japan word by learning this
and kana is japanese vowel, it is basic stuff that you have to learn first
but kana is not formal word......