Customization

Not easily, no. Everything is possible though. You could write some Javascript to find&replace it, or you could reimplement the same menu using your own wording:

First hide the current one, by adding this CSS:

li#groupby_dropdown {
    display: none;
}

Then reimplement it using your own words:

<li class="dropdown open" id="groupby_dropdown">
      <a class="dropdown-toggle" data-toggle="dropdown" href="#">
          Group by <b class="caret"></b>
      </a>
      <ul class="dropdown-menu">
        <li class="groupby year"><a href="#" onclick="groupby('year')">
            Year</a></li>
        <li class="groupby author"><a href="#" onclick="groupby('author_short')">
            Author</a></li>
        <li class="groupby type"><a href="#" onclick="groupby('type')">
            Type</a></li>
        <li class="groupby keyword"><a href="#" onclick="groupby('keyword')">
            Keyword</a></li>
        <li class="groupby downloads"><a href="#" onclick="groupby('downloads')">
            Downloads</a></li>
      </ul>
</li>

Alternatively, if you can provide the translation, then we can add it as a language option. We don't yet have the option to change the language but this would be as good a start for it than any.