I have two store views, I changed one of them from wesbitetest.com to websitetest.com/fr-ca/ but the front end didn't load the js and CSS files as expected, and the URL becomes websitetest.com/fr-ca/?___store=fr_ca instead of websitetest.com/fr-ca/
Run these commands on your server to make sure that everything is deployed well after that change.
alias mage="php -d memory_limit=-1 -f bin/magento" mage setup:upgrade && mage setup:db-data:upgrade && mage dev:source-theme:deploy && mage cache:flush && mage indexer:reindex & mage setup:static-content:deploy -j 10 -f en_US fr_CA mage setup:di:compile mage maintenance:disable
We need more details to help you efficiently, such as what have you already tried, any errors, any permissions issues, etc.
In the meantime, you can follow standard theme debugging steps:
Make sure your theme settings are scoped correctly, under
Content -> Design -> Configuration
In addition, you will want to clear our your pub/static/frontend/* and var/view_preprocessed/* directories.
rm -rf pub/static/frontend/* var/view_preprocessed/*
You should then try as Abdul suggested: compile, deploy static content, and clear caches.