BeyondCompare from Scooter Software is a very versatile directory and file comparison tool. It allows filters for binary formats, weighting lines and defining a grammar for comparing text files. This is a simple grammar for .po files (used by GNUGetText):
Comment=Text matching ^# and next 0 lines
After defining this grammar, all changes in lines starting with an “#” are considered comments and therefore “unimportant differences” which can be ignored using the “ignore unimportant differences” option in the View menu.