在JavaScript中：

const pattern = /^[A-Za-z\s]+\([A-Za-z]{3,}\)$/;
const str1 = 'United Kingdom (English)';
const str2 = 'Belgium (Dutch)';

console.log(pattern.test(str1)); // true
console.log(pattern.test(str2)); // true

模式说明：

^ asserts the start of the string.
[A-Za-z\s]+ matches one or more letters or spaces.
\( matches the opening parenthesis.
[A-Za-z]{3,} matches three or more letters (assumed to be the inside string you mentioned).
\) matches the closing parenthesis.
$ asserts the end of the string.