I think what Ferdinand de Saussure is saying is that language is a temporarily man-made representation of "pure values" or the actual object/emotion/action/thing. Language that describes the object/emotion/action/thing may change over time, but the object/emotion/action/thing itself will not. For example, just 50 years ago, the Chinese word for dragon was "龍", and after now the word becomes "龙". The character changes but the true value behind the character does not. The reason for the changes in the words may vary, which is why Saussure calls them "momentary arrangements", because the words are just temporarily and artificially made for the convenience of communication.