You can upload existing translations for file formats that support logical strings or they otherwise have clear segmentation rules (i.e. it's possible to match the segment with translation without a manual alignment).
If this language does not exist in your project yet, you need to first add it. If you already have this language in this project, just skip to step 2.
Open Files -tab and scroll down to Translated Languages section.
Click New language, add the language and continue to Step 2.
If you have already this language in your project, just choose "Update" next to the corresponding filename:
You may need to click the target language first if you have multiple files in your project.