There are various ways to import translations from a file.
Here you can find all supported formats.
In the menu of the "Start to translate" card, you can find an "IMPORT FILE" entry.
You will find the IMP (import) button next to the EXP (export) button.
Be sure to click the "save" button.
What does "Keep segments not imported" mean? Disabling that option, means that if the importing file does not include a key, that is already present in your project, it will delete it in locize.
Here you can find more information about special formats like CSV and XLSX.
The "all" format at least includes also a namespace column, so this is a good format to export/import translations of multiple namespaces. Make sure the namespace exists before importing this format.
Also in the focus view you will find the IMP (import) button next to the EXP (export) button. Useful when importing additional translations in an existing namespace.
Be sure to click the "save" button.
If necessary make sure you choose the corresponding language and/or version before doing so.