Fix SQL query to retrieve language setting

Fix SQL query to retrieve language setting

In commit e171f6f (Migrate all DB code to use PDO, 2012-08-08),
PDOStatement::fetchAll() was introduced as a drop-in replacement for
mysql_fetch_array(). However, PDOStatement::fetchAll() returns a list of
all results while mysql_fetch_array() returns a single result only.
Instead of adding the missing indirection, simplify the code by using
PDO::fetchColumn().

Also add some safeguards to prevent warnings if the result set returned
by the query is empty.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>