Localizers usually work on the localizable files without the context of the source files including the localized strings; it is important to add comments to the localizable files themselves, too. These comments are generally referred to as localization notes. There is an established format for those, which is described in this document.
It's important to follow the format as closely as possible. There are a number of automated tools that parse these comments for easier access and use by localizers.
DTD files
<!-- LOCALIZATION NOTE (entity name): comment -->
Properties files
# LOCALIZATION NOTE (key): comment
File-wide comments should use the same format, be at the top of the file (after the license header, though) and just drop the (entity name/key) reference.