Damned if I know why, but the GTK+ installation that comes with the Win32 version of GAIM doesn't handle Hebrew properly (mismatched parentheses, problematic handling of hyphens etc.). I've already mentioned that installing the Glade compilation of GTK+ solves the issue, however I attributed it to a particular broken version of the GAIM installer, which seems not to be the case. I should probably file a bug report about this.