Is there an expression in Chinese for saying "I'm taken"?
You can use 名花有主  to describe a third person 'being taken' in a romantic sense. For example: &quot;他已经是名花有主&quot; (he is already taken)
Since it sounds arrogant to call oneself  &quot;名花&quot; (quite a catch), &quot;名花有主&quot; is only appropriate when describing a third person.
Also &quot;名花&quot; is usually a description of a lady. Using it to describe a man is less common, therefore someone would say &quot;名草有主&quot; instead
Or you can use &quot;心有所屬&quot; to declare you are &quot;taken&quot;
&quot;我已经心有所屬&quot; = (my heart had already settled on someone)
But 心有所屬 is quite a literary phrase, rarely used colloquially.

我已经有女朋友

It sounds most natural in native Chinese speech