The .po template file (project.po) is not considered to be a translation file. In order to create one, copy it to project.es.po, open it in poedit and copy each string to the translation column. You can quickly do this by selecting all strings (click on the first string, scroll to the last string and SHIFT-click on it), then press CTRL+B; save as project.es.po. Then your default language file will be available like any other language file.
The routines that you mention, "SetDefaultLanguage" and "Get DefaultLanguage", are not available. Are you talking of "SetDefaultLang" and "GetDefaultLang" which are in LCLTranslator, not in DefaultTranslator?
I just tested with the translation sample project in the folder "examples/translation" of the Lazarus installation - "GetDefaultLang" returns my country symbol without any issues. Please have a look at this project, it is heavily commented.