If you want to customize your theme then you can use child theme say blank child theme. Simply add the blank child theme in the themes directory and activate the theme. Now only few plugins will be deactivated so you'll have to activate them again and your theme design and settings will remain same and you can customize your theme.
For translation you should use the translation plugins such as codestyling plugin or loco translate plugin.
Refer:
https://wplms.io/support/knowledge-base/translate-using-loco-translate-plugin/
Just make sure the languages files are saved in the wp-content/languages folder, then your languages will be safe from the update and you'll just have to sync the files after update.