Browsing the archives, we've had lots of discussions about Japanese Mega CD games, but it doesn't look like we have a single thread that:
(a) lists all the Japanese exclusives,
(b) describes them a bit,
(c) talks about whether they're playable if you don't speak Japanese,
(d) talks about whether they're exclusive to the Mega CD and/or available elsewhere in English, and
(e) talks about compatibility on US/EU hardware with a hacked BIOS (i.e. whether they have hardcoded region lockout).
So I thought I'd start one. Work in progress! I'll edit the first post as new info comes in, either from your posts or my research. The archived SegaBase site was a big help with the more obscure games.
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Titles in blue are linked to the corresponding Sega-16 review.
A Rank Thunder Tanjouhen (Aランクサンダー 誕生編)
Visual novel/card RPG hybrid. Not really playable without knowing Japanese.
After Armageddon Gaiden (アフターハルマゲドン外伝 魔獣闘将伝エクリプス)
RPG. Unplayable without knowing Japanese. Featured in the Sega-16 article Left in Japan.
Aisle Lord (アイルロード)
RPG. Unplayable without knowing Japanese. Dungeon crawler. Here's a gameplay video.
Alshark (アルシャーク)
RPG. Unplayable without knowing Japanese. Ported from the X68000, and there's also a Turbo CD version (released after the Mega CD release). Featured in Left in Japan. At least one Sega-16 member doesn't think much of it. A walkthrough is available at Gamefaqs. Here's a review at Mobygames and a gameplay video.
Anetto Futatabi (アネット・再び)
Side-scrolling beat-'em-up. Mega CD exclusive. Cutscenes, title screen and closing credits are in Japanese, but completely playable without knowing the language. Works for me (WFM) on US hardware with Everdrive.
Arcus 1-2-3 (アークス I・II・III)
RPG. According to A Black Falcon, this is "a collection of three games originally released separately on several Japanese computers, including the PC-8801, PC-9801, MSX, and X68000." Allegedly playable without knowing Japanese, if you use Zigfried's guide cited in Melf's review. Described as a very lengthy dungeon crawler. Featured in Left in Japan, and reviewed by Melf in March 2012.
Arslan Senki [The Heroic Legend of Arslan] (アルスラーン戦記)
Tactical RPG. Unplayable without knowing Japanese. Racketboy calls it a "turn-based strategy RPG". Featured in Left in Japan. Review at Defunct Games.
Bakuden: The Unbalanced Zone (爆伝 アンバランスゾーン)
A "musical action-adventure game", or that's what this guy says at least. Also on the Turbo CD.
Burai: Hachigyoku no Yuushi Densetsu (ブライ 「八ハチ玉タマのノ勇士ユウシ伝説デンセツ」)
RPG. Unplayable without knowing Japanese. Also on the MSX, Turbo CD, and SNES.
Capcom no Quiz: Tonosama no Yabou (カプコンのクイズ殿様の野望)
According to this site, a historical quiz game. I'd imagine you actually have to be Japanese to play this one.
Captain Tsubasa (キャプテン翼)
Soccer sim. Probably playable for non-Japanese speakers? Or, um, it's been translated into Arabic, if that works for you.
Cosmic Fantasy Stories (コズミック ファンタジー ストーリーズ)
RPG. Apparently somewhat playable without the language. Two games in one, both ported from the PC Engine. Featured in Left in Japan. Cosmic Fantasy 2 was released in English for the Turbo Duo. Review at Mobygames. This is currently being prepared for translation; the script of the first game has been completed and is awaiting insertion!
Cyborg 009 (サイボーグ009)
Platformer. Cutscenes are in Japanese (and allegedly numerous). Playable without the language.
Daihoushinden (大封神伝)
RPG. Unplayable without knowing Japanese. Apparently hard to find. Featured in Left in Japan.
Death Bringer: The Knight of Darkness (デスブリンガー)
RPG. Unplayable without knowing Japanese. Dungeon crawler. Also available on the Turbo CD, PC-9801, and X68000.
Detonator Organ (デトネイター・オーガン)
Graphic adventure. Unplayable without knowing Japanese.
Devastator (デバステイター)
Shooter/platformer. Playable without Japanese.
Dynamic Country Club (aka 3D Simulation Dynamic Country Club) (ダイナミックカントリークラブ)
Ported from Sega System 24, an arcade-style golf game. Playable without the language. Reviewed by Gratuitous Science.
Earnest Evans (アーネスト エバンス)
Platformer. Pretty much identical to the cart version, except with added cutscenes (in Japanese) and Redbook audio. 100% playable w/o Japanese language. WFM on US hardware w/ Everdrive.
Eegawa Suguro no Super League [Super League CD Deluxe](江川卓のスーパーリーグCD)
Baseball game. Sequel to Super League CD (listed below).
F-1 Circus CD(F1サーカスCD)
First-person racing game. Totally different from the top-down cart version, according to this post and A Black Falcon. Also reviewed by Gratuitous Science (scroll down a bit). Looks to be mostly in English.
Faeria (aka Seirei Shinseiki Fhey Area)(精霊神世紀フェイエリア)
RPG. In Japanese, but Racketboy and Left in Japan say it's somewhat playable without language skills. Zigfried has written a walkthrough of the game.
F.H.B. - Funky Horror Band (惑星ウッドストック - ファンキー・ホラー・バンド)
RPG. Unplayable without knowing Japanese. Heavily text-based. A fan-translation may be in the pipeline.
Gambler Jikko Chuushinha 2 (ぎゅわんぶらあ自己中心派2 激闘!東京マージャンランド編)
Mahjongg game. Apparently it's quite a good one if you're into that sort of thing.
Game no Kanzume Vol. 1 & 2 [Sega Games Can Vol. 1 & 2] (セガのかんづめ)
Game collections taken from Sega's Meganet download service, most of which are available elsewhere and/or are easily playable without Japanese. The exceptions are the Phantasy Star text adventures, which have recently been translated.
Genei Toshi: Illusion City (幻影都市)
RPG. Completely unplayable without knowing Japanese. Enhanced port of the MSX original; also released for FM Towns. A translation has been attempted; current status unknown. Featured in Left in Japan. Poster McMasters notes that "A translation of the MSX Turbo R version was released a few years ago but the last 25% of the game was untranslated." Here's a review at Mobygames.
Genghis Khan II: Clan of the Gray Wolf [Aoki Ookami to Shiroki Mejika: Genchou Hishi] (蒼き狼と白き牝鹿 元朝秘史)
Simulation/strategy. Probably not playable without Japanese, but ported to a zillion other systems. That includes an English version on the Genesis/MD, to which this adds Redbook audio and not much else, apparently.
Heavy Nova (ヘビーノバ)
Fighter. Supposedly just the cart version with Redbook audio, more or less, so language shouldn't be an issue. Reviewed on Sega-16 on 2014-01-02. Later released on X68000.
Ishii Hisaichi no Daisekai [Hisaichi Ishii's Political World] (いしいひさいちの大政界)
A political sim? With a title like this, I don't think there's much hope for non-Japanese players.
Jango World Cup (雀豪ワールドカップ)
Mahjongg game. Supports four players, or so I'm told.
Mahou no Shoujo: Silky Lip (魔法の少女シルキーリップ)
RPG. Unplayable without knowing Japanese. Heavily text-based.
Mega Schwarzschild (メガシュヴァルツシルト)
Strategy game set in space, a la Starflight. Probably unplayable without knowing Japanese. Port from MSX and/or PC Engine. Featured in Left in Japan.
Might and Magic III: Isles of Terra (マイト アンド マジックIII Isles of Terra)
RPG. Probably unplayable without knowing Japanese. PC version, in English, is widely available. Featured in Left in Japan.
Night Striker (ナイトストライカー)
Rail shooter. Arcade port, later ported to PSX and Saturn. The whole game is in English.
Ninja Warriors, The (ニンジャウォーリアーズ)
Beat-'em-up. Arcade port. 100% playable in English -- in fact, the whole game is in English, including audio! Only the subtitles for the story scenes are in Japanese. Also on PC Engine (though according to A Black Falcon that version is dramatically inferior), plus multiple computer ports in Europe.
Nobunaga no Yabou: Haouden (信長の野望 覇王伝)
Strategy/sim. Another entry in the long-running series from Koei. Also on the Mega Drive, SNES, and 3DO, but I don't think any of those were translated. If you don't speak the language, it's probably better to stick with one of the localized games in the series.
Nostalgia 1907 (ノスタルジア 1907)
RPG/graphic adventure set aboard an ocean liner in 1907. Probably not playable without Japanese. Ported (?) from PC-98.
Pro Yakyuu Super League CD (プロ野球スーパーリーグCD)
Baseball game. Apparently it's almost the same as the cart version, just with added Redbook audio (and maybe FMVs).
Psychic Detective Vol. 3: Psychic Detective Aya (サイキック・ディテクティヴ・シリーズ Vol.3 アヤ)
Psychic Detective Vol. 4: Psychic Detective Orgel (サイキック・ディテクティヴ・シリーズ Vol.4 オルゴール)
RPG/graphic adventure. Also on FM Towns, PC-98, PC Engine CD, and Windows. The US-released 3DO FMV game (Psychic Detective) is unrelated.
Quiz Scramble Special (クイズスクランブルスペシャル)
The name pretty much says it all. I'm sure it's a lost cause if you don't speak Japanese.
Ranma 1/2: Byukuran Aika [Ranma 1/2: White Orchid Serenade] (らんま1/2 白蘭愛歌)
A mix of story scenes (still pictures + audio dialogue) and...rock-paper scissors battles. Doesn't sound very appetizing, but might be playable for fans of the series?
Record of Lodoss War [Lodoss Tou Senki: Eiyuu Sensou] (ロードス島戦記)
Strategy RPG. Apparently playable without the language, especially for those familiar with the series; that, with a knowledge of katakana, may suffice. Featured in Left in Japan.
Romance of the Three Kingdoms III: Dragon of Destiny [Gekijoban Sangokushi III] (三國志III)
Strategy/historical sim in typical Koei style. Available in English on the Genesis/MD and any number of other platforms. Anything to recommend this one besides the Redbook audio?
Seima Densetsu 3x3 Eyes [3x3 Eyes: Legend of the Divine Demon] (サザンアイズ)
RPG. Unplayable without knowing Japanese. Featured in Left in Japan. The soundtrack apparently has murderous saxophone. Review at Mobygames.
Sengoku Denshou(戦国伝承)
Beat-'em-up. Ported from Neo Geo. Fully playable w/o Japanese. Apparently a dreadful port. Said to be tricky to get working on US hardware, but works for me with US Genesis Model 2, Sega CD Model 2, and BIOS replacement via EverDrive, using the Japanese Model 2 CD BIOS.
Shadowrun (シャドウラン)
RPG. Some say it's unplayable without knowing Japanese, but a knowledge of katakana can apparently go a long way. The Mega CD version is completely different from other platforms (including the Genesis). A translation has been attempted; current status unknown. Featured in Left in Japan. Here's a forum thread with a mini-review of the game and links to the romhacking.net project.
Shin Megami Tensei [New Digital Devil Story / Devil Summoner: The New Adventure] (真・女神転生)
RPG. Unplayable without Japanese. Supposedly much the same as the PC Engine version, and according to Racketboy there's also a Super Famicom version that's been translated. Featured in Left in Japan. Reviewed by Sega-16 in April 2016.
SimEarth (シムアース)
Sim game, ported in English to loads of other platforms (none of them Sega). Anything special about the Mega CD version?
Tenbu: Mega CD Special (天舞メガCDスペシャル)
Strategy/historical sim. Another Nobunaga game, but SegaBase has positive things to say about it. Probably not playable without the language. Enhanced port of a PC-9801 game; also released on Super Famicom.
Tenkafubu: Eiyuutachi no Houkou (天下布武 英雄たちの咆哮)
Strategy/historical sim. Yet another Nobunaga game, but SegaBase has positive things to say about this one too. Probably not playable without the language.
Urusei Yatsura: Dear My Friends (うる星やつら ディアマイフレンズ)
Graphic adventure. Allegedly playable without Japanese, other than not being able to understand the cutscenes. Also discussed a bit on Sega-16 here.
Warau Salesman [The Laughing Salesman] (笑ゥせぇるすまん)
Graphic adventure? Ported from MSX. Probably not playable without the language.
Winning Post (ウイニングポスト)
Racehorse management sim. Part of an acclaimed series by Koei; at least one other entry has been translated into English (for the Saturn), but not this one. Probably a drag without the language.
WonderMega Collection (ワンダーメガ コレクション)
Collection of games, music videos, and other stuff for the WonderMega. Should mostly be navigable without knowledge of the language. Will this run on other hardware?
Yumimi Mix (ゆみみ みっくす)
Menu-based anime FMV game. Playable without the language, but you probably won't have more than a vague idea of what's going on. An enhanced version was released on the Saturn as Yumimi Mix Remix.
REMOVED:Tokyo Mahjong Gakuen(not released, may never have existed)
Sega-16 user xelement5x has researched this game, and based on his findings it appears that it was never released. (Was it ever even a title in development?)