file path fallback for default language (fixes #67)

[yankl]

This PR attempts to resolve #67.

Here is the rationalization:

In the twig file, you can see that a language link is shown only if the show_language variable is not false. (line 26). This variable is set to false if, in particular, translated_page is false and untranslated_pages_behavior == 'hide' (lines 15 and 18). The former variable is set in the line above (14): translated_page = langswitcher.translated_pages[language] This array is defined in the php file, lines 123-135. In line 126, $translated_pages[$language] gets defined as null, and is only redefined (in line 132) under the condition (line 129) that file_exists($translated_page_path). This variable is defined in the previous line as follows: $translated_page_path = $page->path() . DS . $page_name_without_ext . '.' . $language . '.md'; Ignoring the rest of the line, we see that $language is hardcoded into the file path. However, the very point of include_default_lang_file_extension being set to false, is that in this case, for the default language, the language code will not appear in the file path, and so $translated_page_path will necessarily be set to a nonexistent file path and $translated_pages[$language] will remain null, so translated_page will be set to null in the twig file and the default language will never appear in the language switcher (unless it's the active page)!

My patch attempts to fix this by checking if the path with the language extension exists, and, if not, and if the given language is the default language, redefining the path as not including the language extension.

[rhukster]

Closing due to my rewrite for version 3.0