Before assigning the proper language to each client, you will need to enable each of the languages in: Settings / General settings / Languages / Add Language. Then, you can assign the language to the client. 

If it doesn't work, check if all the needed strings are translated to the target language. (In debug mode) Settings / Translations / Application Terms / Translated Terms. Search there the untranslated strings. If not found, try Settings / Translations / Application Terms / Generate missing terms. Then search again on the translated terms. 

You can translate the strings by yourself. However, be careful with the next updates. They can overwrite your translations. Read this webpage for more information on the oficial translations: https://github.com/odoo/odoo/wiki/Translations 

Hi,

Set your default Website Language as you want instead of English Language from Website Settings.

and after that, Change your Current User Language for Send Quote to your Client, 

so once your client clicks on the link from your Email then he/she gets it in another language instead of in English.   


Thanks & Regards,
Sunny Sheth

  

If you want to send the quotation/order in client's language then go to Contact/Customer > there you will see the Language field. Fill up the language for that customer. Let's say you for a customer from Spain you select Spanish but your user language is English.

Now when you will send a quotation/order to that customer, it will be in Spanish.