In literal terms, the phrase "I've never so much as X" is a way of expressing that you have never done X. In practical, conversational terms, it's a way of indicating that you lack a particular skill; because if you had that skill, you'd be able to do X, which is typically a simple example of X.
For a slightly more grounded example, I might say "I've never so much as boiled an egg" to indicate that I have no skill at cooking; boiling an egg being a pretty easy cooking task. In this case, Mrs. Figg is using the same logic to suggest that she's incapable of any magic at all, transfiguring a teabag presumably being a simple example of transfiguration.
Of course, that's doesn't preclude the possibility that she's simply an abysmally poor witch, rather than a squib, which brings us to:
Is Mrs. Figg actually a squib?
There's no indication in canon that Mrs. Figg is capable of even a little bit of magic; we certainly never see her attempt any. However, since we don't see her attempt any, there's also no positive evidence that she's a squib and not simply a very poor witch.
One of the indicators that she isn't magical is that she's unable to see Dementors:
'The Dementors... describe them.'
'Oh,' said Mrs Figg, the pink flush creeping up her neck now. 'They were big. Big and wearing cloaks.'
Harry felt a horrible sinking in the pit of his stomach. Whatever Mrs Figg might say, it sounded to him as though the most she had ever seen was a picture of a Dementor, and a picture could never convey the truth of what these beings were like: the eerie way they moved, hovering inches over the ground; or the rotting smell of them; or that terrible rattling noise they made as they sucked on the surrounding air...
The Order of the Phoenix Chapter 8: "The Hearing"
Contrast this to Neville, one of our few examples of a "such a bad wizard he's mistaken for a Squib" character, who appears to have no problem seeing Dementors. Of course, as alexwlchan points out on a related question, none of this is exactly evidence; it's more of a vague hint.
The biggest piece of evidence actually comes from J.K. Rowling's old website1, where she explicitly says that Mrs. Figg has no ability to perform magic (emphasis mine):
Filch has carved himself a niche at Hogwarts and Arabella Figg operates as Dumbledore's liaison between the magical and Muggle worlds. Neither of these characters can perform magic (Filch's Kwikspell course never worked), but they still function within the wizarding world because they have access to certain magical objects and creatures that can help them
Although the choice of the Bulgarian translator is interesting, it seems to have been nothing more than an error, or else a phrase that just doesn't translate well into Bulgarian.
1 Courtesy of the Wayback Machine