To tag words in templates we use Jinja2 i18n extension and tags {% trans %} & {% endtrans %}. More information in a official Jinja2 documentation. Default language is Russian, but for clean templates it will be better to translate them to English. Translations are stored in locale dir. Good example of tagged templates is index.html.

Tagged text (messages) can be extracted by the bybabel tool (installation). For more info how to use bybabel, read article Internationalization and localization with webapp2.

Cool software for translating and compiling .po catalogs is PoEdit (Widnows/Mac/Linux)