Can a law firm in a country be hired in a legal action for you in another country?
If the firm, or any of its associates, are admitted to practice in the other country, yes, they can.
If a legal aged US citizen marries a legal aged foreigner outside the US is the marriage legal in the US?
He cannot move to this country and begin the practice of law without undergoing a bar examination to qualify him to practice US law. However, any UK-legal" actions he initiates in his country which then come to this country as a legal action are admissable in court, because they were legal and lawful in the country where they were intitated.