Multilingual "Posted on: ... - By: ..." labels I would like to ask how to translate labels in drupal 7 like "Posted on: ... - By: ...".  I also have difficulties with translation for prev-next buttons text in calendar. I created a multilingual website based on Nexus 7.x-1.3 theme.
The Nexus 7.x-1.3 theme didn't considered internationalization for some of its  templates:
To enable it to translate "Posted on: ... - By: ...", edit the node.tpl.php on line 90.

&lt;ul class="meta clearfix"&gt;
  &lt;li&gt;&lt;strong&gt;Posted on:&lt;/strong&gt; &lt;?php print $date; ?&gt;&lt;/li&gt;
  &lt;li&gt;&lt;strong&gt;By:&lt;/strong&gt; &lt;?php print $name; ?&gt;&lt;/li&gt;
&lt;/ul&gt;


you will need to use the t() function to change the lines to:

&lt;ul class="meta clearfix"&gt;
  &lt;li&gt;&lt;strong&gt;&lt;?php print t('Posted on:'); ?&gt;&lt;/strong&gt; &lt;?php print $date; ?&gt;&lt;/li&gt;
  &lt;li&gt;&lt;strong&gt;&lt;?php print t('By:'); ?&gt;&lt;/strong&gt; &lt;?php print $name; ?&gt;&lt;/li&gt;  
&lt;/ul&gt;


I'm not sure about which calendar you mean, in case that you are using the one bundled with Date module (Date popup), I believe those buttons can be translated using the Translate interface (admin/config/regional/translate/translate).