Update upgrade-from-v1.md

Text amended as "A new method" was referring to two. CHanged the text to "New methods".

Fix translation french

A bit less stricter check as suggested by @qiang: https://github.com/yiisoft/yii2/commit/2aa8fc71714134feac3f0a09dd61837d920be534

po file generating added.

Files are generated using message/extract command.
E.g.: ./yii message/extract message/config.php po

Fixes #1959 : compare model attribute with value/uncheck options

init of advanced application now allows to specify answer for overwriting files  via init --overwrite=n

Fixes #1965: `Controller::findLayoutFile()` returns incorrect file path when layout name starts with a slash

MongoDB Cache class

Update core-code-style.md

Array short

Update i18n.md

Correction name ru_RU zh_CN to ru-RU zh-CN

Add French Translation