Hello! There is no rule that the values should/must be entered in British English. As long as only one language is mentioned, no language code is used and it is automatically the language typically used in the country. This applies to areas that have free values, such as names and addresses. 

For keys that have only a few values, such as highway, local languages are not desirable, as the data evaluators (and that includes the map creators) do not have the knowledge of all languages.