Hello,
In order to translate anything in your theme or in plugins you should turn to the GNU gettext localization framework used by Wordpress(read more in the codex).
Thus, if you need to translate a text string you should use:
<?php
// This returns the translated string for use in PHP
__('Translate me');
// This echoes out the string
_e('Translate me');
?>
So, to translate parameters of the_content you should pass them like so:
<?php the_content(__('Read more...', 'your-theme-textdomain-name')); ?>
if you need to pass additional parameters, you have to separate them properly:
<?php the_content('<p class="readmore-link">' . __('Read more...', 'your-theme-textdomain-name') . '
'); ?>
and then use poEdit (or similar tool) to translate your theme. And ofcouse don't forget to load the theme textdomain in your functions.php file:
<?php load_theme_textdomain('your-theme-textdomain-name'); ?>
Good luck :)