Gee-whiz, nickel.
Tons of good answers here. Bet, you're experiencing option paralysis by now.
Well, not to be outdone--given the vast variety of English dialects--let me add some American lingo for you too.
In the first sense of
τζάμπα μάγκας, what you'll hear nationwide is
false bravado.
Alternatively, as pontios already suggested, you can add just about anything you like after the word
armchair: +hero, +rebel, +quarterback, +general, +warrior, etc.
In the second sense of the term (=big talker), as dominotherory pointed out, there is no single term across America.
Off the top, I'd say
all hat and no cattle, a phrase my wife has never heard of since usage varies by state.
Here's an assortment of synonyms from
Wiktionary:
all hat and no cattle
(US, idiomatic) Full of big talk but lacking action, power, or substance; pretentious.
Synonyms:
all bark and no bite; all bluff and bluster; all booster, no payload; all crown, no filling; all foam, no beer; all ham, no let; all hammer, no nail; all icing, no cake; all lime and salt, no tequila; all mouth and no trousers; all mouth and trousers; all shot, no powder; all sizzle and no steak; all talk; all talk and no action; all wax and no wick; all motion and no meat; all show, no go; all fart and no poo
Good luck!