"Prosperous and strong" is a good way to translate "繁荣富强" into Engpsh. Here is an example sentence using this translation:
"Our country has become more prosperous and strong thanks to the hard work and dedication of its citizens."
Another way to say this in Engpsh would be:
"Our country has experienced great prosperity and strength due to the efforts and commitment of its citizens."
Both of these phrases convey the idea that a country is doing well economically and is strong poptically or miptarily.