Magento2 Checkout – How to Validate Zip Code Field

You either need to define a new validation type with the correct regex to match the Unicode characters or customize the existing validation type. I believe that you will need to use an expression like p{L} instead of a-zA-Z; more info can be found at http://www.regular-expressions.info/unicode.html#prop.

To tell you how to accomplish the customization I had to go way back (literally, to archive.org's WayBack Machine capture of the support page for the class) to remind myself about extensible changes to the validation.js class:

To add your own validator do this:

Validation.add('class-name', 'Error message text', function(value [, element]) {
     return /* do validation here */ 
}, options);

or this:

Validation.add('class-name', 'Error message text', options);

The first example above includes a function as the third argument. The function enables you to write your own custom validation. The options argument is optional. The second example the third argument has become the options argument. Validator options can be used to perform common validation options without the need to write them into a function. Multiple options can be combined to create a complex validator and they can also enhance your custom validation function. Here are the available options and example usage below:

Validation.add('class-name', 'Error message text', {
     pattern : new RegExp("^[a-zA-Z]+$","gi"), // only letter allowed
     minLength : 6, // value must be at least 6 characters
     maxLength : 13, // value must be no longer than 13 characters
     min : 5, // value is not less than this number
     max : 100, // value is not more than this number
     notOneOf : ['password', 'PASSWORD'], // value does not equal anything in this array
     oneOf : ['fish','chicken','beef'], // value must equal one of the values in this array
     is :  '5', // value is equal to this string
     isNot : 'turnip', //value is not equal to this string
     equalToField : 'password', // value is equal to the form element with this ID
     notEqualToField : 'username', // value is not equal to the form element with this ID
     include : ['validate-alphanum'] // also tests each validator included in this array of validator keys (there are no sanity checks so beware infinite loops!)
});

Of particular note for your needs:

For example here's one of the in-built ones:

Validation.add('validate-alpha', 'Please use letters only (a-z) in this field.', function (v) {
     return Validation.get('IsEmpty').test(v) ||  /^[a-zA-Z]+$/.test(v)
});

You should be able to change the class from "validate-zip-international" to "validate-number" to add Magento's frontend JS validation for numbers, or "validate-digits".

Source: /js/prototype/validation.js for a list of all validation classes (addAllThese method)

['validate-number', 'Please enter a valid number in this field.', function(v) {
    return Validation.get('IsEmpty').test(v) || (!isNaN(parseNumber(v)) && !/^\s+$/.test(parseNumber(v)));
}],
['validate-digits', 'Please use numbers only in this field. Please avoid spaces or other characters such as dots or commas.', function(v) {
     return Validation.get('IsEmpty').test(v) ||  !/[^\d]/.test(v);
}]