So hey, welcome to ハロー!プロジェクト Wiki! And that is the first thing I will start with. I wasn't sure whether to go with the katakana like that, make Wiki katakana as well, or just use Hello! Project Wiki. After all, the official H!P logo just uses Hello! Project. But at a glance it seemed like katakana + Wiki was a bit of a norm for J Wikia names, so I went with that. Still not 100% about it, though, and it's possible to be changed.
Most page section names and infobox section names I took from J Wikipedia. Occasionally Google Translate was brought into play. I tried then searching for real-world uses of the translated phrase to see if I was being lied to. Still, it's possible something bad has gotten through--if you know better, pleae say so.
Dates are formatted to the Japanese 2012年5月2日 format. Times are formatted to Japanese 13分30秒 format, those numbers being minutes and seconds. I realized after I'd started that that not even the official H!P site bothers with that format, but it's what Wikipedia uses. Again, something that could be changed--and better now while the wiki only has a few dozen pages.
It's obvious from discography pages that 通常盤 is used for regular editions, 初回生産限定盤 for limited. However, in some places I tried to be a bit shorter and am pretty sure I didn't always find the best solution. If a discography page wants to say something applies to multiple editions, it will usually go all the way. 初回生産限定盤A/初回生産限定盤B/初回生産限定盤C or whatever. I've shortened it to 初回生産限定盤A-C. In a very few places I went even shorter. On English pages we'll use shorthands like R, LA, LB, etc. I don't see an obvious way to shorten down the 7 kanji of "Limited Edition" to one character like "L", so I just flat out used the LA, LB version. Open to suggestions.
For characters that aren't kana or kanji, I'm generally trying to stick with the same ones we'd use on the English site. () instead of （）. LOVE instead of ＬＯＶＥ. !! instead of ！！. And a few others. I wasn't a stickler at first, until I realized Wikia cares. 有頂天LOVE and 有頂天ＬＯＶＥ point to different pages. Same with 夢見る 15歳 versus 夢見る １５歳. Official pages tend to use both of these in different situations, sometimes even on the same page, so I don't really feel like I'm being unfaithful to the source. As part of this, multi-A-sides are separated by /, even if sometimes official sites list them using ／. Now that I check, it looks like J Wikipedia does this, too.
Song credits are sometimes not as full as those on the English site. For the vast majority of these I just copied the kanji names directly from Tsunku.net, so adding in extra credits would take more translation and time to find other people's Kanji names--significantly more work than I was looking to put into those sections at this time.
The favicon for the site is the same design as the one for the English site, but with the relative strengths of R/G/B changed. To be honest my main purpose here was so that I could easily differentiate between J and E H!P wiki tabs in my browser, but I think it looks pretty alright.
The background image is from the same random generator as that used to make the English wiki's background, but doesn't have the site name (because as stated above I was unsure about it), and doesn't have the start year (because "SINCE 2014" isn't impressive).
The artist templates are the older ArtistTemplateTemplate type, rather than the fancier type Maopyon has replaced the major groups with. Just simpler and faster for me to get results, and I haven't touched any of the concert stuff anyway.
Category name formats mostly based on what Wikipedia was using, but category relationships mostly based on the English H!P wiki. Equivalents of "2002 Singles" or "Minimoni Albums" are included, but not stuff I didn't have something good to copy from, like "6 Members Line-Up" or "Last Single to Feature a S/mileage Member".
Pages and categories have always had the English interlanguage link added as they're created. Robo Kiss is occasionally told to put the reverse ones in place at the English site.
I've prettied up image names a bit. You'll see covers are generally something like EnglishName-la.jpg or EnglishName-ev.jpg. Not Japanese, but hey. File names like 4908-u45niog.jpg suck, so I'm just trying to avoid that as long as possible.