First game had a re-release which added a new campaign and rehauled some of the gameplay; I haven't been able to try it out for myself yet though.
in terms of gameplay differences, I'd say the main one is there are now several secret missions and you can unlock mobile armors to use. some of them are not good at all, some of them are very ridiculous and just obliterate....everything....
the main difference is probably the translation. you know how in the snes translation, Roid/Royd/Lloyd is pretty angry? He gets really pissed off, he screams, he has PTSD. supplemental sources go further and say he's an alcoholic and abuses drugs. He has a line that's like, "I can think of nothing of your death!!!!"
Those are very extreme words, for a video game protagonist to say.
In the DS translation he's kind of just like this strict army commander that gets pissed off sometimes when things go wrong. It's weird for him to be written that way, considering what happened to him, and it's especially weird since (front mission 2 spoilers)
you meet him in FM2 and he's so addled/jaded/pissed off/manipulated because of everything that has happened to him and how he's dealt with it that he is now fighting for the main atagonist of the entire series
there's also another campaign, where you play as a UCS officer, Kevin Greenfield. It's more challenging than the OCU campaign, which is not sayhing much since there is nothing challenging in the OCU campaign, but Kevin himself is not interesting. He's just a bad Gundam protagonist.
Last time I checked that same team was preparing to release the same for the second game, but I haven't checked in there for a while, so I don't really know what came of that.
It's good. The events that happen outside of combat are translated, but not inserted in the game. So there's a .txt file containing those script bits, but everything in battles is translated. I recommend it.
.: I wondered why Front Mission 2 and 5 was not released here. Oh well.
After looking through the Front Mission Evolved gameplay, I can see why people disliked it.
this series has been treated very poorly. when square enix released fm 1st in the US, they did zero marketing. I had no idea it came out or existed until three years afterwards. like my old kung fu master once said, "in order to sell a game, people have to know about it.....idiot...."
the other thing is, in japan front mission is kind of like this serial drama series. There's ALL these supplemental things, like radio productions, and mangas, (novels too, I think???) and they're ALL directly connected. for example, front mission 5's plot is a lot more satisfying if you've played fm1 and 2, because characters are constantly referring to/gossiping about the events of those games. FM3/4 as well, just not as much. So it kind of makes sense to not release a game if people won't understand it.
But then again even if you've not played any other front mission game, front mission 5 is one of the best strategy games ever made, and one of the best games in general.