The `.po` files are generated files, the messages are created from the code (and a few other places) mechanically. Editing the message strings of the `.po` file will just get overwritten.
As the comment above says it comes from `geany.glade.h` which is itself a generated file from `geany.glade` which is edited by the glade UI editor. So thats where it has to be edited. Note that as stated in the `HACKING` file you have to use Glade 3.8.5.