Investigating this has revealed a problem…
First, the key you want to be editing is admin_js.tagging.choose_for_topic, not js.tagging.choose_for_topic.
Second, you can remove the discourse_tagging plugin because it has been moved into core Discourse.
The reason the string is in admin_js is because we have been adding strings to the admin_js section when we shouldn’t have. With the admin_js.admin section collapsed, the client.en.yml file looks like this:
Oops.