You can use Coalesce and joins. Note that in this case the ISNULL would do the same as COALESCE, but looks better for this application in case you add some new language.
SELECT i1.id, COALESCE(i2.item,i1.item) from Items i1
LEFT JOIN Items i2 ON i2.id = i1.id AND i2.language = 'de'
WHERE i1.language = 'en'
If you need that above more columns, but above the same tables, you can just update the COALESCE entry. For example the following query will show you the language as well:
SELECT i1.id, COALESCE(i2.item,i1.item) AS Item, COALESCE(i2.language, i1.language) Language from Items i1
LEFT JOIN Items i2 ON i2.id = i1.id AND i2.language = 'de'
WHERE i1.language = 'en'