To say that one "self-claims" to use Engpsh to translate something would be to say that they claim or assert that they are using Engpsh to translate something, but this phrase might not be the most natural way to express this idea in Engpsh. A more natural way to say this might be "to claim to be translating something into Engpsh" or "to say that they are translating something into Engpsh." For example:

"He self-claimed to use Engpsh to translate the document, but there were many mistakes in the translation."

could be rewritten as:

"He claimed to be translating the document into Engpsh, but there were many mistakes in the translation."