I would like to know the right translation and saying "紙界"(Paper World) in Japanese. Should it be "pepa kai", "kami kai" or something else?
Google translate "紙" as "Kami", and "Kami" as "神" when reverse. People also use "ペーパー"(Pepa) as Paper. I'm a bit confused.
Update
I also found "和紙" (washi), "洋紙" (youshi), and "千代田紙" (Chiyodashi). Does it mean "紙" also called "shi"?
Should it be "pepa kai", "kami kai" or something else?
I think that you are going to get a variety of answers. I think that in this case, you will actually be better using the katakana version of the English phrase 'Paper World,' which is ぺーパーワールド. Using the Japanese 界 may be confusing, as かい (kai) could be interpreted as one of many things, including large bodies of water.
Not only does the katakana form avoid potential misunderstandings (Japanese people will understand these words I think), but the English element also adds a little bit of a coolness factor that you don't get with something that is more Japanese.
Google translate "紙" as "Kami", and "Kami" as "神" when reverse. People also use "ペーパー"(Pepa) as Paper.
紙: This is the Japanese word for paper, it is read as 'kami'.
ペーパー: This is a Japanese cognate word that is used for combination words like 'Paper towel' (ペーパータオル), paper plate (ペーパープレート), and other words like this. You won't use this to describe a single sheet of paper, or a ream of paper.
神: This is also read as 'kami', but it actually the character for the word god. This will vary between religious contexts, but for the most part, it is used in the most, if not all, of the same contexts English speakers use the word 'god' (with or without capitalizing the 'G').
I digress. There are many cases where different characters have the same reading. I mentioned one case above with the reading for the world character, the sea character, and others. In this particular case, かみ (kami) could be 紙 (paper), 髪 (hair), or 神 (god). That's why Google translate does not work in reverse very well.
Things like this are also why I would never recommend that a beginning Japanese student use Google translate. Trust the textbooks, and avoid the translating tools like Google translate until you are at more of an intermediate level. If you want an online dictionary, try jisho.org
I also found "和紙" (washi), "洋紙" (youshi), and "千代田紙" (Chiyodashi). Does it mean "紙" also called "shi"?
し is another reading for 紙. You will discover as your studies progress that kanji characters have multiple readings. The reading depends on context, but as you are a beginner, I wouldn't recommend worrying about it just yet. For now, I think it is important to know that these differences exist. When you start to expand on those studies, you can start digging deeper then.
If this "Paper World" is an existing Western company name, you usually have to leave it untranslated or use katakana ペーパーワールド. (Note that there is already a company with the same name.) Unlike Chinese which tries to convert every foreign name into kanji, Japanese people usually just use Latin alphabet or katakana for branding of foreign names. You should not ignore this tradition unless there is a really good reason.
If you absolutely need a kanji name, 紙世界 (kami sekai) is a possibility, but people would probably guess the company is mainly about traditional Japanese paperwork like origami.
紙界 (kami kai?) sounds like an unfamiliar made-up word, and people usually imagine it's an imaginary world where everything is made of paper (like Paper Mario). 紙の界 sounds very strange, regardless of the purpose, because 界 is a suffix but not a standalone word.
For historical reasons, Japanese has tons of homonyms, and most kanji have more than one reading. But you can (or should) forget about kanji at least in this case.
This is just the expansion of my comment, but I can suggest an alternative way of translation of world as 国 ("land; country; kingdom"), if you only mean that "a place where things of a kind gather". While 世界 is the likely translation for that word in most cases, it bears a nuance of a "self-contained environment" that has its own collection of history, ecosystem, laws, etc. from scratch, in other words, something like "universe" when we refer to a fictional work.
So I think 国 should be much more modest and fitting when you intend a place where you can see all sorts of novel paper sheets rather than this kind of world.
