Along with Antionette's very useful link, there are some more below. In the modern Royal Navy (RN), the "senior" lieutenant is referred to as the Executive Officer (or sometimes and historically as the First Lieutenant or Number One).

You could just render this as "Lieutenant" - and/or include the "of the watch" bit, which is more relevant for historical purposes in the RN.

Either way you might want to include the Dutch as well.

The wikipedia reference also provides links to the US navy.

http://en.wikipedia.org/wiki/Lieutenant_commander

http://www.royalnavalmuseum.org/info_sheets_nav_rankings.htm

http://www.royal-navy.mod.uk/server/show/nav.1770