Accept-Locale:

[1] Activity & Exercise Logs — Fitbit Web API Docs (2016-10-05 06:00:11 +09:00) <https://dev.fitbit.com/docs/activity/>

Request Headers
Accept-Locale optional The locale to use for response values.

[2] Fitbit Web API Basics — Fitbit Web API Docs (2016-10-05 06:00:11 +09:00) <https://dev.fitbit.com/docs/basics/>

Some of the API responses include text fields that may be suitable for displaying to the end user. Setting the Accept-Locale header will return a translated response if available. We currently support the following locales:
Accept-Header Locale
en_AU Australia
fr_FR France
de_DE Germany
ja_JP Japan
en_NZ New Zealand
es_ES Spain
en_GB United Kingdom
en_US United States (default)

[3] Fitbit Web API Basics — Fitbit Web API Docs (2016-10-05 06:00:11 +09:00) <https://dev.fitbit.com/docs/basics/>

API calls reveal and log resource values in one of the unit systems based on the value of the Accept-Language header. If an endpoint respects the Accept-Language header, it is explicitly mentioned in the endpoint details.
Accept-Language Unit System
en_US US
en_GB UK
none of the above or not provided METRIC