import json

f = '/home/szymon/projects/tfb/app/panel/public/resources/lang/page/fr/b2b.json'
with open(f) as fh:
    d = json.load(fh)
d['prepare_offer'] = 'Préparer un devis'
with open(f, 'w') as fh:
    json.dump(d, fh, ensure_ascii=False, indent=2)
    fh.write('\n')
print('ok:', d['prepare_offer'])