select only one element from an priority ordered group with limits?
Without window functions, the query will be complicated. One way to solve the problem in MySQL:

SELECT 
    n.*
FROM 
    ( SELECT DISTINCT featureName
      FROM i18n
    ) AS d
  JOIN
    i18n AS n
  ON  n.id = 
      ( SELECT i.id
        FROM i18n AS i
        WHERE i.featureName = d.featureName
          AND i.locale IN ('en', 'de', 'de_AT')
          AND i.priority &lt;= 3
        ORDER BY i.priority DESC
        LIMIT 1
      ) 
ORDER BY 
    n.featureName ;           -- or whatever you like, eg:  BY n.id


Also note that the above query will fail it is in SQL Server (which uses TOP, not LIMIT) and possibly fail in other DBMS as well.