Yes, nopAccelerate supports nopCommerce multi-lingual store. Moreover, depending on your language you may also need to modify Solr Config file for performing appropriate language analysis. Click here to check the list of languages supported by Apache Solr for Language Analysis.
Note that depending on a release and version you are upgrading from / to, these steps may vary. However, these are generic steps which should work in most cases. Please follow following steps to upgrade your plugin.
Make a backup of your website, plugins, database, Solr Configuration and Indexing Data
Update Solr Config & Schema files with the new file and restart your Tomcat or Servlet Container you are using. (Make sure to migrate your changes made to Solr Config & Schema file.) (Note that, not all plugin update may require this).
Uninstall the nopAccelerate Solr plugins
Upgrade plugins with new files including new View Files (need to copy to your current active theme and website View folder)
After upgrade plugins, Install the nopAccelerate Solr plugins
Activate the Plugin (your old settings will be preserved, so you do not need to configure them again)
Perform a full indexing (This may only required if there are changes in Solr Schema file)
No. Installation does not require any down-time on your site. However, during installation you may need to restart your application. Your site will work normally also at the time of performing full Solr indexing.