Craft 3 Multi Language Setup
First of all you should remove all "language" subfolders from your web directory because your web server will route directly in those instead of your default web/index.php. Craft will determinate the CurrentSite depending on your url not on the location of the file.

Second: make sure to insert an array with aliases in your config/general.php 
I'll show a "complex" solution for multiple asset sources for multiple sites. If your site shares the same resources you'll just need the first array element but I want to show the concept

$publicPath = realpath(dirname(__DIR__) . '/web');
// ....
'aliases' =&gt; [
    'assetPath1'    =&gt; $publicPath . DIRECTORY_SEPARATOR . 'assets' . DIRECTORY_SEPARATOR . 'folder1' . DIRECTORY_SEPARATOR,
    'assetPath2'    =&gt; $publicPath . DIRECTORY_SEPARATOR . 'assets' . DIRECTORY_SEPARATOR . 'folder2' . DIRECTORY_SEPARATOR,
    //....
]


You should never access those files with 

&lt;script src="`{{ siteUrl }}/assets/folder1/jquery.js`"&gt;


Because this will use the current site url and append the structure. Therefore you have the alias function

&lt;script src="`{{ alias('assetPath1') }}/assets/folder1/jquery.js`"&gt;


Now you can do some fancy stuff like

&lt;script src="`{{ alias('assetPath' ~ currentSite.id) }}/assets/folder1/jquery.js`"&gt;


Or whatever identifier you like to access your asset files dynamically.