The flags in Ryu's stage of World Warrior are the Fūrinkazan, the battle standard of 16th century daimyo Takeda Shingen. The standard is quoting chapter seven of Sun Tzu's The Art of War and the full translation is:
Let your rapidity be that of the wind, your compactness that of the forest.
In raiding and plundering be like fire, be immovable like a mountain.
This creed is simplified by the four kanji: wind, woods, fire, mountain. These symbols also appear on Ryu's belt in some official artwork, including in Street Fighter IV.
The setting for the fight with Vega in all iterations of Street Fighter II is Mesón de la Taberna, as seen beyond the cage and above the painting of the bullfight in the background. The name translates tautologically as 'Inn of the Tavern'. The barroom is underground and probably in Barcelona given that one of Vega's specials is the 'Flying Barcelona Attack'. Curiously, a cropped version of this stage appears in the data for Street Fighter II and is never used in the game. Being the same width as the bonus stages it has created speculation that there was originally intended to be a fourth bonus game, taking place after the Vega fight.
Events in the real-world sometimes need reflecting in Street Fighter as seen when selecting Fei Long in Hyper Street Fighter II: The Anniversary Edition, released in 2003. The colonial flag of Hong Kong was changed to the flag of China in recognition of the handover that took place in 1997. In Street Fighter IV the current flag of Hong Kong depicting a flower is used, as it was for the characters Yun and Yang in Street Fighter III. Similarly, Zangief now fights under the Russian flag; in Street Fighter II the flag of the U.S.S.R. is used throughout, in every revision, even though the Soviet Union was dissolved on Boxing Day in 1991. This might be due to the announcer; if Capcom wished to change 'U.S.S.R.' to 'Russia' they would have had to re-record the sound code for this. When updating the status of Hong Kong this was not a complication.
A spectral image of Dhalsim is visible in the box art for NTSC/PAL releases of Street Fighter II: Turbo. The artist, Mick McGinty, (who also did box art for World Warrior and Special Champion Edition) was following a brief from Capcom to include the figure but once the logo text was overlaid Dhalsim's appearance became heavily obscured. On the original copy, however, it is much clearer. It is appropriate that Dhalsim should appear to be teleporting in or out of the scene as it was in this game; Hyper Fighting a.k.a Turbo that his moveset was expanded to include this ability.
On a similar note, provided you're watching on a widescreen television, Akuma does make a cameo appearance in Street Fighter II: The Animated Movie. Get ready to pause just after Ryu walks into Calcutta.... In the fight between Dhalsim and E. Honda that follows soon after, Dhalsim remarks that he can no longer compete since he feels, 'A power... but where? Emanating from a man!' He's referring to Ryu of course, but with Akuma in the vicinity one might say that Dhalsim's qi is being disturbed by a double dose of Ansatsuken! A further bit of trivia: this depiction of Akuma, apparently selling fruit, was reprised by Bengus in an illustration for Gamest's Warrior's Dreams bonus issue.
Street Fighter II's African-American boxer, designed as a pastiche of real-life boxer Mike Tyson, is named M. Bison (the 'M' standing for 'Mike') in Japanese versions of Street Fighter. When World Warrior was localised for the international market, the names of the boss characters (except for Sagat) were shuffled as the name and resemblance could have led to a likeness infringement lawsuit, particularly as Tyson had an interest in the videogame market around this time via the Mike Tyson's Punch-Out!! title on the NES. The likeness was progressively toned down with every subsequent Street Fighter II release. By the time of Alpha 3 in 1998, however, Capcom dared to hint at the connection again with this Balrog win quote, referring to the infamous Holyfield-Tyson II incident the year before: 'If you fight like that again, I'll bite you ear off!'
Following on from the above, the characters known as M. Bison, Vega and Balrog internationally are Vega, Balrog and M. Bison, respectively, in Japan. Confusion when discussing these characters across regions has led to the nicknames Dictator, Claw and Boxer being adopted online. Minor confusion also stems from the fact that Akuma is named Gouki in Japan and Charlie is Nash, leading to the logical conclusion that his full name is Charlie Nash (as found in the Udon comics and implied in the Street Fighter II V television series—now confirmed by Capcom). The Nash 'mix-up' stemmed from the translator Tom Shiraiwa doing a literal translation of the Japanese (i.e. 'Nash') only for this to be rejected by Capcom U.S.A. on the grounds that Nash isn't an English first name. Hence Charlie.