<DButton
@class="flw-header-btn"
@id="flw-create-account-header-btn"
@action={{route-action "showLogin"}}
@label="Already_have_an_account?_Login"
/>
renders as this

any help would be appreciated.
You are attempting to use a string. But I don’t believe that’s supported.
You should be providing a localisation key which is linked to a string.
See:
Just look inside a popular plugin for guidance on how to do this in a plugin.
In general always look for examples of what you are trying to do in existing source code.