Isn't flash actually a movie format?
Anyway, you dont use any external language to write flash games. You get a copy of Adobe Flash (and not just the free player, though there may be a free 3rd party flash toolkit that I don't know about), draw things in it then code their behaviour in a language called ActionScript which is pretty close to JavaScript (last time I checked was flash4 or something).
ActionScript is just a little scripting language that is used in Adobe Flash. You cannot do anything in flash with C++, Java etc. Java applets can be an alternative to flash for writing website based games. For example, Desktop Tower Defence is Flash using Flash's internal scripting language ActionScript, whereas runescape is a game written using a Java Applet (for the client only).
Although being able to use JavaScript would help you in writing the ActionScript, it is not needed. You could probably learn ActionScript then JavaScript equally well.