Steps to create Multilingual Community

1. Set the default language: This setting is not associated with the default language settings in your Salesforce org. By default, English is the language in which community content displays. 
1.a. In the community on the left sidebar, click on the gear icon and select Languages.

1.b. Select a language in the Default Community Language.

2. Add Language to community: Click on the Add Language button and select the languages you wanted to add to your community and save your changes.

once save you can select any the following settings

2.a. Active on Live Community: This checkbox controls whether the language is available to community visitors in the language selector.

2.b. Fallback Language: Controls which language to display if the selected site language is unavailable.

2.c. Display Label: This setting define the display label for each language. The display label appears in any language picker components you add to your community and in the language selector in Community Builder.

3. Export the content for translation: Click on Export Content and select the language you want to export for translation. Save the .xml file and send it to your translation service.

sample .xml file:

4. Import translated content: Once translation service completed the translation, import a translated .xml file back to community by clicking Import Translation.

Once import is complete test your community pages to make sure the content displays correctly.

Reference: Salesforce Communities Doc