SELECT *
  FROM common_table
 WHERE language_id = 1

SELECT *
  FROM common_table
 WHERE language_id = 2

If EXISTS (SELECT * FROM common_table WHERE language_id = 1)
BEGIN
    SELECT * FROM common_table WHERE language_id = 1
END ELSE BEGIN
    SELECT * FROM common_table WHERE language_id = 2
END

SELECT * FROM common_table
         WHERE language_id = 2 and
               0 = ( SELECT count(*) FROM common_table
                                     WHERE language_id = 1
                   ) ;

Select ...
From common_table
Where language_id = 1
    Or  (
        language_id = 2
        And Not Exists  (
                        Select 1
                        From common_table
                        Where language_id = 1
                        )
        )

SELECT *
  FROM common_table
 WHERE language_id = 1
UNION
SELECT *
  FROM common_table
 WHERE language_id = 2
       AND NOT EXISTS (
                       SELECT *
                         FROM common_table AS T2
                        WHERE T2.language_id = 1
                      );