(Page numbers in brackets refer to the single-volume unabridged German edition. All English translations marked [DS] are my own.)
Spengler is full of witty barbs :
Nothing is simpler to do than to establish a system in place of thoughts that one does not have. Yet a good thought is of little value, if it is spoken from an empty head. [DS]
I think Spengler is not an advocate of literal translation:
Wir sprechen in Sätzen und nicht in Worten. 
We speak in sentences and not in words. [DS]
Perhaps it would be in spirit to translate this as “The way that can be spoken of is not the constant way,” so as not to seem too literal!