I installed Magento2 without composer. I would like to install a few extensions and another language but all tutorials I have seen use composer.
Is it possible to "add" composer when Magento is already installed ? How do I have to proceed ?
It's not possible to install extensions with composer if Magento was not installed with composer. So you should start over with a fresh composer install.
Good news is, you can copy all the non-core files from app
and then still use the same database, so nothing is lost.
The answer is Yes, you can install it after Magento2 is installed.
First try before to check in console if you have a composer in your server like this:
composer --help
If command help displays, Composer is already installed else if you have an error message it means that it is not instaled
It's time to install it like this:
curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer
Nb: make sure that you're connected as root