xgettext picks up comments from the source

[Caolán McNamara]

In case you're unaware of it, when extracting translation strings from
our source we are using xgettext --add-comments which is "place all
comment blocks preceding keyword lines in output [po] file". And these
appear in pootle (just double checked that).

e.g.

// To translators: This is the a paper size
NC_("RID_STR_PAPERNAMES", "Letter"),

So, to help translators out you can stick a comment above the line in
the .hrc (there is similar functionality in glade for the .ui files
too).