I have changed the front-end language, Now my site looks different. It seems that the css is lost.. How can I change the language without loosing my style of front-end ?
In Magento 2, static view files like the CSS are generated per theme and locale.
Let's say you switched to
de_DE, then you need to generate the files with
bin/magento setup:static-content:deploy de_DE
If you set your system to developer mode, you should not need to do this manually.