I translated to Spanish the readme files of «Powie's WHOIS» and also the .po file using for the setting page. But this last strings are not listing on WordPress.org repository. I think the reason is the text domain. I verfied the files of the plugin and find the text domain you used: pwhois, but you must to change it to the slug of the plugin: powies-whois. This is the method WordPress uses to autodetect the text domain for plugins and themes.
I also wanted to suggest you change some strings for better undestanding:
Show on free domain > Show on available domains
Show on connected domain > Show on unavailable domains
Domain is free > Domain is available
Domain is not free > Domain is unavailable
pWhois Usage > Powie's WHOIS Usage
pWhois Pro > Powie's WHOIS Pro
Powies WHOIS Settings > Powie's WHOIS Settings
Powies WHOIS Setup > Powie's WHOIS Setup
I can fix the strings if you want, just tell me and will be a pleasure to do it.