I started learning Japanese and it quickly became clear where that accent comes from. This comment is about the mechanics, as I understand them, so skip if you dgaf.

Most of their consonant sounds are paired with a vowel sound that follows, eg: ta (tah), te (teh), ti (tee), to (toe), tu (too), though they aren't always audibly pronounced (eg, in Naruto, Sasuke is the spelling, but it's pronounced like Saskeh). That's where the "su" sound sometimes replacing an "s" sound at the end of words comes from, or "ru" replacing an "r" sound. It's correct with and without audibly pronouncing the "u", so Japanese speakers might add or omit it based on preference.

They also don't have all of the consonant sounds we do. Most notable is their lack of an "R" or "L" sound, but they do have a sound that is like a mix of the two. Sasuke's voice actor pronounces "Naruto" with that sound instead of an "R" sound. It's like an R with a slight roll, not as pronounced as in French, but from making an R sound and briefly touching your tongue to your teeth as if you were making an L sound.

They are also missing the V sound, their closest would be the B sound. Their word for GPS navigator is "Nabi", for example.

And they have so many loanwords from other languages that they even use a seperate alphabet (katanaga) for them. It's a one-to-one translation from their other alphabet (hiragana). Though even two alphabets wasn't enough and there's kanji on top of that, which is another set of over a thousand symbols that help disambiguate their many words that are spelled the same but pronounced differently (basically which syllable the rise in pitch changes to a drop in pitch).

Also, their sentence structure is very different. Like a typical english sentence might go: Subject verb object. Jaoanese sentences are more like: Subject object verb, though, like English, their grammar allows for many variations, and also omissions. Like they can drop the subject entirely from the sentence. Like I could introduce myself as "Buddahriffic desu", but I could introduce you as "SaraTonin desu". A direct translation would be "SaraTonin is" or "Buddahriffic is" and you'd need to figure out who the subject is using context.

The end result is that I'm impressed with any Japanese person who can speak english well enough to communicate, let alone if they are fluent, because it's a lot more than I was able to do with theirs, unless the necessary communication is very basic.

Oh one more tidbit: the Japanese use "ne" (neh) similarly to how Canadians use "eh", which works like adding a "right?" to the end of a statement (or an audible extra question mark to a question).