LanguageManager.guessLanguage

Picks a classLanguage for given file name and content type, according to the information in lang files.

Either filename or content_type may be null. This function can be used as follows:

GtkSourceLanguage *lang;
GtkSourceLanguageManager *manager;
lm = gtk_source_language_manager_get_default ();
lang = gtk_source_language_manager_guess_language (manager, filename, NULL);
gtk_source_buffer_set_language (buffer, lang);

or

GtkSourceLanguage *lang = NULL;
GtkSourceLanguageManager *manager;
gboolean result_uncertain;
gchar *content_type;

content_type = g_content_type_guess (filename, NULL, 0, &result_uncertain);
if (result_uncertain)
  {
    g_free (content_type);
    content_type = NULL;
  }

manager = gtk_source_language_manager_get_default ();
lang = gtk_source_language_manager_guess_language (manager, filename, content_type);
gtk_source_buffer_set_language (buffer, lang);

g_free (content_type);

etc. Use gtksource.language.Language.getMimeTypes and gtksource.language.Language.getGlobs if you need full control over file -> language mapping.