Since with version 3.8.0 it is no longer necessary to overwrite the complete configuration file, but only the affected configurations, it is sufficient to create a corresponding snippet, e. g. <magento-install-dir>/app/etc/configuration/operations.json, which has the following content

{
   "operations":{
      "ce":{
         "catalog_product":{
            "validate":{
               "plugins":{
                  "subject":{
                     "id":"import.plugin.subject",
                     "listeners":[
                        {
                           "plugin.process.start":[
                              "import.listener.reset.loader.eav.attribute.option.value"
                           ],
                           "plugin.process.success":[
                              "import.listener.stop.validation"
                           ]
                        }
                     ],
                     "subjects":[
                        {
                           "id":"import.subject.validator",
                           "create-imported-file":false,
                           "number-converter":{
                              "locale":"de_DE"
                           },
                           "file-resolver":{
                              "prefix":"product-import"
                           },
                           "listeners":[
                              {
                                 "subject.artefact.header.row.process.start":[
                                    "import.listener.validate.header.row"
                                 ]
                              }
                           ],
                           "params":{
                              "custom-validations":{
                                 "sku":[
                                    "/.+/"
                                 ],
                                 "product_type":[
                                    "simple",
                                    "virtual",
                                    "configurable",
                                    "bundle",
                                    "grouped",
                                    "giftcard"
                                 ],
                                 "visibility":[
                                    "Not Visible Individually",
                                    "Catalog",
                                    "Search",
                                    "Catalog, Search"
                                 ]
                              }
                           },
                           "observers":[
                              {
                                 "import":[
                                    "import_product.observer.composite.base.validate"
                                 ]
                              }
                           ],
                           "callbacks":[
                              {
                                 "sku":[
                                    "import.callback.custom.regex.validator"
                                 ],
                                 "store_view_code":[
                                    "import.callback.store.view.code.validator"
                                 ],
                                 "attribute_set_code":[
                                    "import.callback.attribute.set.name.validator"
                                 ],
                                 "product_type":[
                                    "import_product.callback.validator.product_type"
                                 ],
                                 "tax_class_id":[
                                    "import_product.callback.validator.tax.class"
                                 ],
                                 "product_websites":[
                                    "import.callback.store.website.validator"
                                 ],
                                 "visibility":[
                                    "import.callback.visibility.validator"
                                 ],
                                 "related_skus":[
                                    "import_product.callback.validator.link"
                                 ],
                                 "upsell_skus":[
                                    "import_product.callback.validator.link"
                                 ],
                                 "crosssell_skus":[
                                    "import_product.callback.validator.link"
                                 ],
                                 "created_at":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "updated_at":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "special_price_to_date":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "special_price_from_date":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "custom_design_to":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "custom_design_from":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "new_to_date":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "new_from_date":[
                                    "import.callback.validator.datetime"
                                 ],
                                 "price":[
                                    "import.callback.validator.number"
                                 ],
                                 "special_price":[
                                    "import.callback.validator.number"
                                 ],
                                 "map_price":[
                                    "import.callback.validator.number"
                                 ],
                                 "msrp_price":[
                                    "import.callback.validator.number"
                                 ],
                                 "qty":[
                                    "import.callback.validator.number"
                                 ]
                              }
                           ]
                        }
                     ]
                  }
               }
            },
            "add-update":{
               "plugins":{
                  "subject":{
                     "id":"import.plugin.subject",
                     "subjects":[
                        {
                           "id":"import_product.subject.bunch",
                           "number-converter":{
                              "locale":"de_DE"
                           },
                           "file-resolver":{
                              "prefix":"product-import"
                           },
                           "params":{
                              "copy-images":false,
                              "media-directory":"pub/media/catalog/product",
                              "images-file-directory":"var/importexport/media/catalog/product",
                              "update-url-key-from-name":true,
                              "clean-up-variants":false,
                              "clean-up-links":false,
                              "clean-up-media-gallery":true,
                              "clean-up-empty-image-columns":true,
                              "clean-up-website-product-relations":true,
                              "clean-up-category-product-relations":true,
                              "clean-up-empty-columns":[
                                 "special_price",
                                 "special_price_from_date",
                                 "special_price_to_date"
                              ]
                           },
                           "observers":[
                              {
                                 "import":[
                                    "import_product.observer.composite.base.add_update"
                                 ]
                              }
                           ]
                        }
                     ]
                  }
               }
            }
         }
      }
   }
}

In the example above, we override the validation as well as the add-update operation, as both need the appropriate number format. As this changed the number format to locale de_DE the file MUST contain numbers like 1.408,88.