A simple alternative to maintaining a custom translation file is to just filter any untranslated strings. (This is also a general possibility for text that you want to change, but that’s a different topic.) Generally, the only two elements you need to worry about are custom fields and emails.
For handling custom form fields, there is a basic code snippet example using the wpmem_register_form_rows filter. The example code snippet allows you to filter any specific custom fields based on the current text domain. How to use it is explained in the code comments of the example code.
The second code snippet is for handling emails. What it does is create a custom email for each language you identify in the snippet (explained in the code comments). Now you can create an email for each language in the plugin’s Emails tab.