Sure, it definitely makes more sense to keep your files separated like that... Beats putting all of the language files together, and sifting through it... Plus, it makes more sense, so you can always refer to the same page (sign_up.php, or thank_you.php) and just change the language designation...
In the end, it all comes down to your preferences... If this is what works best for you, then go for it... You are the one who has to make changes later, so do what makes the most sense and works best for you.