have a look at a plugin on my plugins list activity to home that may help you 

Each plugin may also have its own language files. BTW, you shouldn't alter core files, but instead make a new plugin and override those translations.

@steve_clay I created a new directory mod>titles>lenguages and file es.php

<?php
 
    $spaniss = array(  
            'activity' => 'Actividad',
    );
 
    add_translation("es",$spanish);
?>

but the problem continues

@sz1hosting "This plugin is invalid: String could not be parsed as XML"

hmm works for me on szsocial.com take a look, maybe liang lee can advice you  on that

SOLVED: I changed "Activity" title directly in the elgg file en.php, I think an plugin not lets run translations title.

Either in profile settings dont translate the plugin title's

Thanks  @steve_clay @sz1hosting