This "Pro License" is a Lifetime license, you only need to pay once. and you can use Pro forever.
30-day refund Guarantee! no question asked.
This desktop software can translate .srt .ass .vtt file.
Don't worry, This software also has a free version.
You can download the free version on http://tern.1c7.me/
The free version has 1 million characters/month limitation, which is a generous amount.
How much is 1 million?
1 million ~= 25 movies (1 hour to 2 hours)
1 million ~= 50 TV Show (20 to 60 minute)
1 million ~= 333 Youtube Video (1 to 5 minute)
1 million ~= 166 Youtube Video (5 to 10 minute)
if you only use this once a while,
Free version probably good enough for you.