"Glè mhath! Meal do naidheachd!"
Translation:Very good! Congratulations!
In English, very well isn't congratulatory. The expression can be neutral, like "ok" or dismissive, like "oh well." It wouldn't be used with a positive response meaning job well done. I can't speak for Gàidhlig but I can definately speak for American English, its context and nuance.
Meal = Enjoy Do = your Naidheachd = News Nothing mentioning happy or birthday in there. It is commonly said to people on their birthday so yeah it kind of replaces happy birthday as I don't think there is a gaidhlig expression for that except from sona chola-breth or something like that but it's used much less commonly than meal do naidheachd, however translating meal do naidheachd as happy birthday would be wrong as it is used for many different occasions. Occasions where we would say congratulations in english