OWM Weather uses the WordPress language, that why you see English output. See the FAQ “In which languages is the weather data displayed?”:
OWM Weather is fully translated into Chilean, French, German, Russian, Spanish, and Serbian. Partial translations are available in Estonian, Italian, Hungarian, Hebrew, Polish, Danish, and Portuguese. The plugin will use these languages if they correspond with the language set in WordPress under General settings.
Open Weather Map offers data in many languages through the “OpenWeatherMap Language” setting within the weather post. This has no influence on the plugin language. If set to Default, the system will prioritize the WordPress site language and resort to English if the site language is unavailable.
You need to set your WordPress language to Serbian in order for OWM Weather to use that language.