Can I translate all my storefront text in a spreadsheet instead of one by one?

Yes. Open Plugins, then Multi Language, and go to the Translations tab. Pick the language, click Export CSV (or Sample CSV for a template), and fill in the value column. Then use Import to upload it. Only keys that already exist in the catalog are imported; any unknown keys are skipped and listed in a report you can download. Product, category, page and blog content is translated on the Content tab.