Basically, There are 2 language files to create to translate the component or module, one for the frontend, one for the backend.
These files have to be named following the convention:
[language-code]-[COUNTRY-CODE].mod_ajax_contact.ini
For example: en-GB.mod_ajax_contact.ini for english, pt-BR.mod_ajax_contact.ini for Brazilian Portuguese.
They have to be placed in the folder corresponding to the language and the frontend/admin part, for example:
Read more: How to translate or modify field labels and error messages?