Change specific block look & feel I have a block like this:

&lt;section id="block-locale-language" class="block block-locale clearfix" data-thmr="thmr_23"&gt;
&lt;h2 class="block-title"&gt;Languages&lt;/h2&gt;
&lt;ul class="language-switcher-locale-url" data-thmr="thmr_8"&gt;&lt;li class="nl first"&gt;&lt;a href="/nl/user/1" class="language-link" lang="nl" data-thmr="thmr_9"&gt;Nederlands&lt;/a&gt;&lt;/li&gt;
    &lt;li class="en active"&gt;&lt;a href="/user/1" class="language-link active" lang="en" data-thmr="thmr_10"&gt;English&lt;/a&gt;&lt;/li&gt;
    &lt;li class="fr last"&gt;&lt;a href="/fr/user/1" class="language-link" lang="fr" data-thmr="thmr_11"&gt;Français&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;




(From the language switcher module)

But now I would like to have something like this:

&lt;div class="btn-group"&gt;
&lt;button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown"&gt;
  Action &lt;span class="caret"&gt;&lt;/span&gt;
&lt;/button&gt;
&lt;ul class="dropdown-menu" role="menu"&gt;
  &lt;li&gt;&lt;a href="#"&gt;Action&lt;/a&gt;&lt;/li&gt;
  &lt;li&gt;&lt;a href="#"&gt;Another action&lt;/a&gt;&lt;/li&gt;
  &lt;li&gt;&lt;a href="#"&gt;Something else here&lt;/a&gt;&lt;/li&gt;
  &lt;li class="divider"&gt;&lt;/li&gt;
  &lt;li&gt;&lt;a href="#"&gt;Separated link&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;




Where do I have to change this?
Create a template file in your theme.

block--locale.tpl.php