Hi Saskia! Could you first confirm whether you have a site hosted here on WordPress.com with the Business plan? That’s the only plan where you can install plugins, but I don’t see any sites on your account with that plan.
While we don’t test themes against Yoast SEO or WPML plugins, I wouldn’t foresee any issues with the SEO plugin. WPML can be harder to predict; sometimes theme settings in the Customizer settings are not translatable, no matter the plugin. The Business plan is refundable within 30 days of purchase, so if there were any issues, you could get a full refund. You could also set up a quick localhost test install first, to check.