Implement i18n middleware and add example.

examples/i18n_example.py

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+"""
+Internalize your bot
+
+Step 1: extract texts
+    # pybabel extract i18n_example.py -o locales/mybot.pot
+Step 2: create *.po files. For e.g. create en, ru, uk locales.
+    # echo {en,ru,uk} | xargs -n1 pybabel init -i locales/mybot.pot -d locales -D mybot -l
+Step 3: translate texts
+Step 4: compile translations
+    # pybabel compile -d locales -D mybot
+
+Step 5: When you change the code of your bot you need to update po & mo files.
+    Step 5.1: regenerate pot file:
+        command from step 1
+    Step 5.2: update po files
+        # pybabel update -d locales -D mybot -i locales/mybot.pot
+    Step 5.3: update your translations
+    Step 5.4: compile mo files
+        command from step 4
+"""
+
+from pathlib import Path
+
+from aiogram import Bot, Dispatcher, types
+from aiogram.contrib.middlewares.i18n import I18nMiddleware
+from aiogram.utils import executor
+
+TOKEN = 'BOT TOKEN HERE'
+I18N_DOMAIN = 'mybot'
+
+BASE_DIR = Path(__file__).parent
+LOCALES_DIR = BASE_DIR / 'locales'
+
+bot = Bot(TOKEN, parse_mode=types.ParseMode.HTML)
+dp = Dispatcher(bot)
+
+# Setup i18n middleware
+i18n = I18nMiddleware(I18N_DOMAIN, LOCALES_DIR)
+dp.middleware.setup(i18n)
+
+# Alias for gettext method
+_ = i18n.gettext
+
+
+@dp.message_handler(commands=['start'])
+async def cmd_start(message: types.Message):
+    # Simply use `_('message')` instead of `'message'` and never use f-strings for translatable texts.
+    await message.reply(_('Hello, <b>{user}</b>!').format(user=message.from_user.full_name))
+
+
+@dp.message_handler(commands=['lang'])
+async def cmd_lang(message: types.Message, locale):
+    await message.reply(_('Your current language: <i>{language}</i>').format(language=locale))
+
+
+if __name__ == '__main__':
+    executor.start_polling(dp, skip_updates=True)

examples/locales/en/LC_MESSAGES/mybot.po

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+# English translations for PROJECT.
+# Copyright (C) 2018 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2018.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2018-06-30 03:50+0300\n"
+"PO-Revision-Date: 2018-06-30 03:43+0300\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language: en\n"
+"Language-Team: en <LL@li.org>\n"
+"Plural-Forms: nplurals=2; plural=(n != 1)\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 2.6.0\n"
+
+#: i18n_example.py:48
+msgid "Hello, <b>{user}</b>!"
+msgstr ""
+
+#: i18n_example.py:53
+msgid "Your current language: <i>{language}</i>"
+msgstr ""
+

examples/locales/mybot.pot

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+# Translations template for PROJECT.
+# Copyright (C) 2018 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2018.
+#
+#, fuzzy
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2018-06-30 03:50+0300\n"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: LANGUAGE <LL@li.org>\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 2.6.0\n"
+
+#: i18n_example.py:48
+msgid "Hello, <b>{user}</b>!"
+msgstr ""
+
+#: i18n_example.py:53
+msgid "Your current language: <i>{language}</i>"
+msgstr ""
+

examples/locales/ru/LC_MESSAGES/mybot.po

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+# Russian translations for PROJECT.
+# Copyright (C) 2018 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2018.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2018-06-30 03:50+0300\n"
+"PO-Revision-Date: 2018-06-30 03:43+0300\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language: ru\n"
+"Language-Team: ru <LL@li.org>\n"
+"Plural-Forms: nplurals=3; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && "
+"n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2)\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 2.6.0\n"
+
+#: i18n_example.py:48
+msgid "Hello, <b>{user}</b>!"
+msgstr "Привет, <b>{user}</b>!"
+
+#: i18n_example.py:53
+msgid "Your current language: <i>{language}</i>"
+msgstr "Твой язык: <i>{language}</i>"
+

examples/locales/uk/LC_MESSAGES/mybot.po

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+# Ukrainian translations for PROJECT.
+# Copyright (C) 2018 ORGANIZATION
+# This file is distributed under the same license as the PROJECT project.
+# FIRST AUTHOR <EMAIL@ADDRESS>, 2018.
+#
+msgid ""
+msgstr ""
+"Project-Id-Version: PROJECT VERSION\n"
+"Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
+"POT-Creation-Date: 2018-06-30 03:50+0300\n"
+"PO-Revision-Date: 2018-06-30 03:43+0300\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language: uk\n"
+"Language-Team: uk <LL@li.org>\n"
+"Plural-Forms: nplurals=3; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && "
+"n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2)\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=utf-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Generated-By: Babel 2.6.0\n"
+
+#: i18n_example.py:48
+msgid "Hello, <b>{user}</b>!"
+msgstr "Привіт, <b>{user}</b>!"
+
+#: i18n_example.py:53
+msgid "Your current language: <i>{language}</i>"
+msgstr "Твоя мова: <i>{language}</i>"
+