All this options are.... improvisations. Bootstrap already offers a solution for this. If you want to change the placeholder for all your dropdown elements you can change the value of
noneSelectedText in the bootstrap file.
To change individualy, you can use TITLE parameter.
example:
<div class="form-group">
<label class="control-label col-xs-2" id="country">Continent:</label>
<div class="col-xs-9">
<select name="zones[]" class="selectpicker" title="All continents" id="zones" multiple >
<option value="Africa">Africa</option>
<option value="Asia">Asia</option>
<option value="North America">America North</option>
<option value="South America">America South</option>
<option value="Antarctica">Antarctica</option>
<option value="Australia">Australia</option>
<option value="Europe">Europe</option>
</select>
</div>
</div>