Hey Stephan,
first of all, thank you for the purchase!
All strings in Ikebana have been wrapped into respective functions according to WordPress internationalization standards, which means that they should be translatable by any third-party plugin that uses these functions. I haven’t used Polylang before, yet from its documentation I can see that it also makes use of the said functions. This page of the documentation might help you with translating the pre-set strings; if it doesn’t please provide access to your back-end by sending login and password to support@satoristudio.net (no disclosure guaranteed) – and I will troubleshoot the issue.
Sincerely,
Andrii / Satori Studio