Expression referring to assuming different positions based on what is convenient at the moment
We have an expression in English 'The Vicar of Bray'. You can look up the whole story but the gist of it is when he says :
We have principles, Sir ! (And if you don't like them, we have others.)
So I would suggest the word unprincipled.
Or 'as unprincipled as the Vicar of Bray.'
Unprincipled
He's a chameleon.
chameleon noun
...
1.2 A person who changes their opinions or behaviour according to the situation.
voters have misgivings about his performance as a political chameleon
[ODO]
The example ODO gives shows how the behaviour may be referred to.