Lokalise supports over 400 languages as defined by ISO codes, however you can set a custom ISO code for exporting the language.
Changing language ISO code
If your project requires non-standard codes for languages, these can be easily set by clicking small triangle next to the language name in projects view and choosing "Change ISO...".
Copying keys to translations
Some file formats are structured to use base language translation as the key name. In order to use inline translation suggestions, Lokalise needs base language translation as value, not key. Use "Copy keys to translations..." function from the same dropdown to actually copy keys to values for the base language.