Regarding the broken language strings when enabling the Filter extension, I have found the cause, which is in the mail extender.
FreeFlarum uses custom mail extender which conflicts with Filter's one for sending mails whenever a post is filtered. When you use default mail settings, the language breaks up, however when you setup a custom mail settings, the extension works fine without breaking anything. I'd suggest using a different code to send mails in this extension such as other extensions use which don't conflict with FreeFlarum. Until then, FreeFlarum users that want to use the Filter extension should override the default mail settings with a custom ones, or just don't use the extension.
More information and troubleshooting about the language breaking issue can be found here, if you don't know about it already.