@pamtbaau:
GitHub - getgrav/grav-premium-issues: Official Grav Premium Issues repository to report problems or ask questions regarding the Premium products offered. · GitHub
Yes — this works through Grav's standard translation system, it's not Premium-specific.
Any string rendered via the |t filter (Twig) or $grav['language']->translate() resolves against the language files for the active page language, so /en/ and /de/ pick up their respective values automatically. The placeholder in the template already uses |t, so you only need to define the key.
Add it to user/languages/de.yaml and user/languages/en.yaml (these override anything shipped by the plugin and survive updates):
yaml
# user/languages/de.yaml
PLUGIN_YETISEARCH_PRO:
SEARCH: "Suche..."
yaml
# user/languages/en.yaml
PLUGIN_YETISEARCH_PRO:
SEARCH: "Search..."
One catch worth flagging: if a key is undefined, |t returns the key string itself rather than null, so a chained |default(...) won't fire — which is why an untranslated field shows the raw PLUGIN_YETISEARCH_PRO.SEARCH. Defining the key in the language files is what fixes it.
This is the same mechanism for free and premium plugins alike, so the multilanguage setup you're describing is fully supported.