Change: treat languages as finished, if translations are 75% completed.

Unfinished translations are not auto-picked from the locale.
In release builds, unfinished translations are not offered in the GUI.
Unfinished translations are available in non-release builds, or by editing openttd.cfg.

src/language.h

@@ -58,6 +58,7 @@ struct LanguagePackHeader {
 	char cases[MAX_NUM_CASES][CASE_GENDER_LEN];     ///< the cases used by this translation
 
 	bool IsValid() const;
+	bool IsReasonablyFinished() const;
 
 	/**
 	 * Get the index for the given gender.