(OUTDATED - see comment below)
How to test this:
1. Enable Show Navigation in Joomla Article options
2. Open plugins/content/pagenavigation/tmpl/default.php
3. Replace JText::_('JPREV') with $row->prev_title
4. Replace JText::_('JNEXT') with $row->next_title
5. Check that when you open an article in the frontend, the buttons at the bottom have the text of the next/previous article.

I think it would be nice if we could choose what to show in the plugin params.
Have it default to the current prev/next behavior and change the default layout accordingly.

Then this PR would be great!

Thanks for your suggestion, I have added the plugin param.

So new testing instructions are:
1. Enable Show Navigation in Joomla Article options
2. Go to Plugin Manager -> Content - Page Navigation
3. Check that changing the option "Link text" does change the page navigation links below an article.

Tested and works as described. Nice work!

When I first read the title of this PR, I expected the option to be able to do something like 'Next' versus 'Next: Second Blog Post'. What are your thoughts on something like that? I do like this option as well, so I'm fine either way.

PS: Also a for consistent implementation of the chevron.

Before:

After:

Please alpha-order the language strings.

When I first read the title of this PR, I expected the option to be able to do something like 'Next' versus 'Next: Second Blog Post'. What are your thoughts on something like that?

I think that the arrows are enough to indicate that the links are pointing to the next/previous post. So I would prefer only the title.

@infograf768 Was there a special reason we used the language strings JGLOBAL_GT and JGLOBAL_LT for this? Maybe RTL languages?

I just checked the arabic language pack and it doesn't have a different symbol in the JGLOBAL_GT and JGLOBAL_LT strings:
./ar-AA.ini:328:JGLOBAL_GT=">"
./ar-AA.ini:357:JGLOBAL_LT="<"

So I think it should be safe to replace them with the icons.

Issue in RTL
Before:

After:

So instead of <?php echo '<i class="icon-chevron-left"></i> ' . $row->prev_label; ?> use something like

<?php $lang = JFactory::getLanguage();
$direction = $lang->isRTL() ? 'right' : 'left';
echo '<i class="icon-chevron-' . $direction . '"></i> ' . $row->prev_label;

(not tested)

I've fixed the RTL issue, can you please test again?

Yep, looks fine to me now.
Setting RTC as we have two tests.

Merged into 3.4-dev. Thanks!