The language on the maps themselves depends on the OpenStreetMap data. As a general rule, names of streets and places are written in the local language. Very often you will see two variants – the local name and the name in your device’s language. You can always add the name in your language directly at OpenStreetMap.org.
We also use transliteration into Latin for certain countries if a name on the map doesn't have translation into your device language. For example, if your device language is English, transliteration will be used in China for all names untranslated to English. You can always disable the feature in the app's settings → General settings → Transliteration into Latin.