Hello,

The placeholder text can be translated/customized by using a XML language file. Create any XML file such as Forms.xml in the /Lang folder of your solution. The following snippet is an example of how you can override the default placeholder text of the Selection element for the English language. 

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<languages>
	<language name="English" id="en">
		<episerver>
			<forms>
				<viewmode>
					<selection>
						<!--Update the values here to override-->
						<selectanoption>-- Select an option --</selectanoption> 
						<selectoptions>-- Select options --</selectoptions>
					</selection>
				</viewmode>
			</forms>
		</episerver>
	</language>
</languages>

I do not have the Portugues language configured so that's why I'm using English as an example. However to do this for Portugues simply update the values in the <language> element. You can find more details about the translation files and how to use them in the documentation here:
https://docs.developers.optimizely.com/content-cloud/v11.0.0-content-cloud/docs/localizing-the-user-interface

Cheers 😄