The following code is not meant to run within Odoo, but as simple scripts:

1. First, we query the list of installed modules via XMLRPC:

#!/usr/bin/env python3
from xmlrpc.client import ServerProxy

db = 'odoo11'
user = 'admin'
password = 'admin'
uid = ServerProxy('http://localhost:8069/xmlrpc/2/common')
    .authenticate(db, user, password, {})
odoo = ServerProxy('http://localhost:8069/xmlrpc/2/object')
installed_modules = odoo.execute_kw(
  db, uid, password, 'ir.module.module', 'search_read',
  [[('state', '=', 'installed')], ['name']],
  {'context': {'lang': 'fr_FR'}},
)
for module in installed_modules:
  print(module['name'])

2. Then, we do the same with JSONRPC:

import json
from urllib.request import Request, urlopen

db = 'odoo11'
user = 'admin'
password = 'admin'

request = Request(
  'http://localhost:8069/web/session/authenticate',
  json.dumps({
    'jsonrpc': '2.0',
      'params': {
        'db': db,
        'login': user,
        'password': password,
      },
  }).encode('utf8'),
  {'Content-type': 'application/json'})
result = urlopen(request).read()
result = json.loads(result.decode('utf-8'))
session_id = result['result']['session_id']
request = Request(
    'http://localhost:8069/web/dataset/call_kw',
    json.dumps({
      'jsonrpc': '2.0',
      'params': {
        'model': 'ir.module.module',
        'method': 'search_read',
        'args': [
          [('state', '=', 'installed')],
          ['name'],
        ],
        'kwargs': {'context': {'lang': 'fr_FR'}},
      },
    }).encode('utf8'),
    {
        'X-Openerp-Session-Id': session_id,
        'Content-type': 'application/json',
    })
result = urlopen(request).read()
result = json.loads(result.decode('utf-8'))
for module in result['result']:
  print(module['name'])

Both code snippets will print a list of installed modules, and because they pass a context that sets the language to French, the list will be in French if there are translations available.