Localise validation messages and placeholders in vue code

[nuest]

Examples:

• validation message: https://github.com/codecheckers/ojs-codecheck/blob/main/resources/js/Components/CodecheckRepositoryList.vue#L71
• placeholder in input field: https://github.com/codecheckers/ojs-codecheck/blob/main/resources/js/Components/CodecheckManifestFiles.vue#L8