How to display an alert message if the value is blank in the form

I have a form (bootstrap) in the script and need it if the user does not select the language and the value "" is blank to display the alert message and that he had to choose a language to proceed on the next step. (as you can see the empty value in the form: value"" represent "Select Language")

This is the form code:

<label for="recipient-name" class="col-form-label">Language:</label>
                                <input type="text" class="form-control langinput" id="country" value="" readonly>
                                <select class="form-control langselect" aria-label="Language Translate Widget" id="country" style="display: none">
                                    <option value="">Select Language</option>
                                    <option value="English">English</option>
                                    <option value="Afrikaans">Afrikaans</option>
                                    <option value="Albanian">Albanian</option>
                                    <option value="Amharic">Amharic</option>
                                    <option value="Arabic">Arabic</option>```