Yes, it is possible of course.
G'MIC doesn't use an usual translation file for this task, and translating the interface means modifying some code, but it is quite simple to do, once we get the idea.
Just to show how it works, here is translation code for the Catalan language :
// Catalan translation
if (!std::strcmp(get_locale(),"ca")) {
if (!s) {
static const char *const ns = "<b>No ha estat possible establir una connexió a Internet !</b>\n\n"
"<i>No es possible arribar a aquestes fonts de filtres :</i>\n";
return ns;
}
_t("G'MIC for GIMP","G'MIC per al GIMP");
_t("<i>Select a filter...</i>","<i>Selecciona un filtre...</i>");
_t("<i>No parameters to set...</i>","<i>Sense par\303\240metres...</i>");
_t("<b> Input / Output : </b>","<b> Entrades / Sortides : </b>");
_t("Input layers...","Capes d'entrada...");
_t("None","Cap");
_t("Active (default)","Actiu (predet.)");
_t("All","Tots");
_t("Active & below","L'activa i les de sota");
_t("Active & above","L'activa i les de sobre");
_t("All visibles","Totes les visibles");
_t("All invisibles","Totes les invisibles");
_t("All visibles (decr.)","Totes les visibles (decr.)");
_t("All invisibles (decr.)","Totes les invisibles (decr.)");
_t("All (decr.)","Totes (decr.)");
_t("Output mode...","Mode de sortida...");
_t("In place (default)","A la capa actual (predet.)");
_t("New layer(s)","Nova/es capa/es");
_t("New active layer(s)","Nova/es capa/es actius");
_t("New image","Nova imatge");
_t("Output preview...","Previsualitzaci\303\263 de sortida...");
_t("1st output (default)","1era imatge (predet.)");
_t("2nd output","2ona imatge");
_t("3rd output","3era imatge");
_t("4th output","4rta imatge");
_t("1st -> 2nd","1era -> 2ona");
_t("1st -> 3rd","1era -> 3era");
_t("1st -> 4th","1era -> 4rta");
_t("All outputs","Totes les imatges");
_t("Output messages...","Missatges de sortida...");
_t("Quiet (default)","Sense missatges (predet.)");
_t("Verbose (console)","Verb\303\263s (consola)");
_t("Verbose (logfile)","Verb\303\263s (arxiu)");
_t("Very verbose (console)","Molt verb\303\263s (consola)");
_t("Very verbose (logfile)","Molt verb\303\263s (arxiu)");
_t("Debug mode (console)","Depuraci\303\263 (consola)");
_t("Debug mode (logfile)","Depuraci\303\263 (arxiu)");
_t(" Available filters (%u) :"," Filtres disponibles (%u) :");
_t("_Maximize","_Maximitzar");
_t("_Restore","_Restaurar");
_t("Update","Actualitzaci\303\263");
_t("Rename","Canviar");
}
If you can send me the same piece of code with the Spanish translation instead, then I'll be able to add it to the plug-in.