Oh, I missed these two words, I will improved in next version.
Now, you can also change it by yourself.
1. Edit single.php, find the following codes
<div class="pagination">
<span class="nav-previous alignleft"><?php previous_post_link( '%link', __( '← Previous Post', 'SimpleKey' ) ); ?></span>
<span class="nav-next alignright"><?php next_post_link( '%link', __( 'Next Post →', 'SimpleKey' ) ); ?></span>
<div class="clearfix"></div>
</div>
2. Change it to:
<div class="pagination">
<span class="nav-previous alignleft"><?php previous_post_link( '%link', __( '← Previous Post', 'OldCar' ) ); ?></span>
<span class="nav-next alignright"><?php next_post_link( '%link', __( 'Next Post →', 'OldCar' ) ); ?></span>
<div class="clearfix"></div>
</div>
3. Open your language file and update it, then you will see “Previous Post” and “Next Post” in the list, just translate them and save the file.
Thanks!