You can maintain a custom translation file for your situation. The upside to doing this is that you have a proper translation of all user facing strings (including custom fields). The downside is that you need to make sure you keep it up to date if any changes are made in the plugin. I try not to make changes affecting translation, but sometimes it is unavoidable.
To maintain a custom translation file, you can start from what is available in your language at the polyglots project. Download (export) the .po file from the site. Then, using a tool like Poedit, you can add your necessary strings and translate them. Poedit will compile this as an .mo file when you save it. The .po is your template file, the .mo is the machine readable compiled file necessary to display translations.
Poedit is free, and there is a premium version as well. You only need the free version to do this, but I recommend the paid version. First, there are additional features; and second, it supports the project. (In keeping with my personal standards, I have a paid version of the program. I don’t recommend products that I don’t own and use myself.)
Once you create a set of custom translation files, you’ll need to do two things:
- Load the files to a location on your web server,
- Set up a filter to tell the plugin to load your custom file instead of the default file.
Custom translation files can live anywhere on your server, but I recommend that you put them somewhere in the wp-content directory. If you use a child theme, that’s a good location that isn’t generally touched. You can also upload via the WP Media Manager and make note of that location. Essentially, do what is right based on your specific WP install as long as they are not in a location that would be overwritten (i.e. not the wp-members folder).
Make sure that your custom file is “wp-members-{locale}.mo”. For example, a standard German is “de_DE” so your file would be wp-members-de_DE.mo (make careful note of the difference between hyphens and underscores – these are critical).
Then set up a filter to load your custom translation file using the wpmem_localization_file filter. This filters the location of the .mo file being loaded. The filter has an added parameter of the current $locale, making it useful for multi-language sites (as long as the plugin locale is correct). You can use the filter in the documentation “as is.” Just make sure to adjust the path based on where you have saved your custom file.