tldr: Open the Python-Fu console (Filters > Python-Fu > Console) and reference GIMP's built-in pdb documentation (Help > Procedure browser). A simple 'hello world' is presented:

img = gimp.image_list()[0]
title_layer = pdb.gimp_image_get_layer_by_name(img, 'Title')
pdb.gimp_text_layer_set_text(title_layer, "Hello world!")