You can absolutely translate the system into your language. We have a guide on using the free Loco Translate plugin to translate any built in texts from OptimizeMember into your language:
OptimizeMember and OptimizeMember Pro are now equipped with support for front-end translation, using standard WordPress® methodologies; i.e. we've implemented the use of _x(), and various other translation routines for many aspects of OptimizeMember's front-end interfaces. For instance, things like Profile panels, Login/Registration Fields, and Pro Form integrations; as well as error messages displayed to Users/Members. Translation support for back-end admin panels provided by OptimizeMember will come in a future release.
Like WordPress® itself, we chose to use the GNU gettext localization framework to provide localization infrastructure for OptimizeMember. GNU gettext is a mature, widely used framework for modular translation of software, and is the de facto standard for localization in the open source/free software realm. If you'd like to translate OptimizeMember and/or OptimizeMember Pro, please use the POT file found inside /OptimizeMember/includes/translations/s2member.pot, which contains all translation entries for both the OptimizeMember Framework (i.e. the free version), and also for OptimizeMember Pro. You translate this one POT file, and that covers both software applications. The file: /OptimizeMember/includes/translations/s2member.pot will be updated with each new release of OptimizeMember.
If this is your first translation of a WordPress® plugin, this article and/or this article, might be of some assistance. When you are finished translating the s2member.pot file, place your completed s2member-[locale].mo file into this directory: /wp-content/plugins/. OptimizeMember will find it there and load it automatically into WordPress® for you. Please feel free to share your translation with the rest of the OptimizeMember community.
Quick Tip: If you only need to translate the front-end of OptimizeMember, please ignore entries in the s2member.pot file with a context matching s2member-admin. Those sections of OptimizeMember are only seen by site Administrators; they are NOT used in OptimizeMember's front-end integration with WordPress®. Skipping over translation entries with a context matching s2member-admin can save you time.