Hi K2A,
 
You can change this message by modifying validation messsge.
 
- Select the contol and on Right side for properties 
- under validation click on the ellipsis next to Pattern,
- The Validation Patterns screen opens. 
- Select an existing Pattern to change message  and click on Edit , 
- It will show you popup to edit the details, 
- Do the changes and click on oK.

 
                
     
                                    
            - Find App-Globalresources directory for Runtime site, normally it is located at C:Program Files (x86)K2 blackpearlK2 smartforms RuntimeApp_Globalresources.
- Make a backup copy of Runtime.resx
- Open it for editing
- Find the following section and change <value> as you may want, for example, type it in a different language:
<data name="RequiredValidationError" xml:space="preserve">
    <value>обязательно для заполнения</value>
    <comment>Required value was left empty</comment>
  </data>