Your chosen language may not come by default. You need to enable it.
Setup → Company Profile → Language Settings → Enable “Enable End User Languages”
Here is help article: https://help.salesforce.com/articleView?id=000182272&type=1. After that you would be able to use this Language for translation.
Use Translation Workbench
The translation workbench allows you to create translations for the customization you’ve made to your Salesforce.com organization. Everything from custom picklist values to custom fields can be translated so your global users can use all of Salesforce.com in their language.
Setup → Translation workbench
- Add your target language in Translation Settings
- Add picklist value for your language using translate section: https://www.greytrix.com/blogs/salesforce/2017/03/28/translate-picklist-into-multiple-languages/
Translation in custom code:
Use custom label for each hard-coded statement/unit of words. And after that translate that in your language. Here issfdc help article: https://help.salesforce.com/articleView?id=cl_translate_edit.htm&r=https%3A%2F%2Fwww.google.co.in%2F&type=5
On user record, update Timezone and Locale.
Last Step — Enable Languages in Community