Detect User Language (Locale) on Google App Engine Python

The best way to guess/detect/determine the locale/language of users is probably by using the Accept-Language field in HTTP request header (see reference [1] for detail). In this post, we will show how to detect/determine user locale according to Accept-Language field in HTTP request header on Google App Engine for Python.

Assume that the website we own support three locales: de (German), en_US (English, United States), and zh_TW (Traditional Chinese). If these three locales matches the locales in Accept-Language field, the website will serve the web pages with the locale of higher preference. If no matches, the website will serve web page with en_US locale (default locale).

The first thing to do to detect/determine user locale is parsing Accept-Language field. In previous post "[Python] Parse Accept-Language in HTTP Request Header" [2], this topic is discussed. Please refer to the post for detail.

Next, the parsed data will be compared with supported locales of the website. The following code snippet demonstrates how to do this:

Detect Locales on Google App Engine Python

def parseAcceptLanguage(acceptLanguage):
  languages = acceptLanguage.split(",")
  locale_q_pairs = []

  for language in languages:
    if language.split(";")[0] == language:
      # no q => q = 1
      locale_q_pairs.append((language.strip(), "1"))
      locale = language.split(";")[0].strip()
      q = language.split(";")[1].split("=")[1]
      locale_q_pairs.append((locale, q))

  return locale_q_pairs

def detectLocale(acceptLanguage):
  defaultLocale = 'en_US'
  supportedLocales = ['de', 'en_US', 'zh_TW']

  locale_q_pairs = parseAcceptLanguage(acceptLanguage)
  for pair in locale_q_pairs:
    for locale in supportedLocales:
      # pair[0] is locale, pair[1] is q value
      if pair[0].replace('-', '_').lower().startswith(locale.lower()):
        return locale

  return defaultLocale

The parseAcceptLanguage function is the same as the function in previous post [2] for parsing Accept-Language field. The detectLocale function compares supported locales of the website with locales in Accept-Language field. Note that the default locale is set to en_US. You can change the defaultLocale and supportedLocales variables to fit the locales design in your website.

Example Usage

If you are using webapp2, pass self.request.headers.get('accept_language'), the variable which contains Accept-Language field on Google App Engine for Python, to detectLocale function and the locale to be served will be returned. The following example will give you idea of what it looks like:

>>> print(parseAcceptLanguage('da, en-gb;q=0.8, en;q=0.7'))
>>> print(detectLocale('da, en-gb;q=0.8, en;q=0.7'))
[('da', '1'), ('en-gb', '0.8'), ('en', '0.7')]

>>> print(parseAcceptLanguage('zh, en-us; q=0.8, en; q=0.6'))
>>> print(detectLocale('zh, en-us; q=0.8, en; q=0.6'))
[('zh', '1'), ('en-us', '0.8'), ('en', '0.6')]

>>> print(parseAcceptLanguage('zh-tw, en-us; q=0.8, en; q=0.6'))
>>> print(detectLocale('zh-tw, en-us; q=0.8, en; q=0.6'))
[('zh-tw', '1'), ('en-us', '0.8'), ('en', '0.6')]

>>> print(parseAcceptLanguage('es-mx, es, en'))
>>> print(detectLocale('es-mx, es, en'))
[('es-mx', '1'), ('es', '1'), ('en', '1')]

>>> print(parseAcceptLanguage('de; q=1.0, en; q=0.5'))
>>> print(detectLocale('de; q=1.0, en; q=0.5'))
[('de', '1.0'), ('en', '0.5')]


[1]Accept-Language used for locale setting
[2](1, 2) [Python] Parse Accept-Language in HTTP Request Header