One major problem Japanese people have with English is the pronunciation.
There are many sounds in English that don't exist in the Japanese language.
Most Japanese people end up replacing those sounds with ones they can pronounce, resulting in the characteristic (and horrible) Japanese-English accent.
My suggestion to teach correct pronunciation is to use IPA (International Phonetic Alphabet).
Most English textbooks don't use IPA to show pronunciation. Most English teachers don't even know IPA. They either rely on memorization, by just speaking the words slowly and hoping the student will memorize the correct pronunciation, or by using the infamous katakana.
IPA in its full format is too complicated for practical uses. The developers of IPA wish to cover all sounds of all languages in the world. For English teaching this in unnecessary.
But it is possible to have a simplified IPA that covers only the English language.
Most dictionaries (especially Oxford dictionaries) already use some form of simplified IPA to show pronunciation.