Há muita discussão, até mesmo nos países de língua inglesa, sobre o uso de “shall” em contratos. Os “pros and cons” praticamente empatam e, s.m.j. (salvo melhor juízo) (ou melhor opção), o uso de “shall” segue “undisturbed”. Para maiores detalhes sobre essa pendenga, veja a opinião de nativos nos links correspondentes:
“In my recent article advocating disciplined use of shall I mention that I haven’t seen any evidence of a flight from shall.” (http://adamsdrafting.com/system/2007/11/30/law-firm-forbids-shall-question-mark)

“I take a position between these two extremes. I recommend that for purposes of business contracts you use "shall" only to impose an obligation on the subject of a sentence -- in other words, to convey the meaning "has a duty to." Eliminating "shall" entirely has little to recommend it: replacing it with "must" would eliminate a useful distinction along with only negligible risks; lawyers find "must" unduly bossy; and "will" is even less promising as an alternative. (http://www.law.com/jsp/ihc/PubArticleIHC.jsp?id=1193043817930)

"Use of "shall", "will", "may", and "should". For mandatory requirements in the specifications use "The Contractor shall". For requirements to be fulfilled by the government use "The Government will". Use "may" to allow an option. Never use "should" in the specifications. (http: //www.swt.usace.army.mil/specs/instructions_2.htm)

"the use of "shall" to mean a requirement and "should" a recommendation." (http://www.ideals.uiuc.edu/bitstream/2142/7572/1/librarytrendsv36i4_ocr.txt)

"...whether we should use will' or shall' in the relevant parts of our speech or writing. ...the future tense required will' for everyone except the first person singular and plural, when the correct use is shall'.
She quotes: "You will have my reply tomorrow" (future tense), but "You shall go to the ball" (determination). http://www.darlingtonandstocktontimes.co.uk/news/1821906.a_day_once_set_aside_for_hirings_firings_and_all_the_fun_of_the_fair/

SHALL: indica “dever”, “obrigação”. Nesse sentido, somente pode ser usado por pessoas, não por objetos. Uma regrinha básica [rule of thumb]: Use “shall” somente quando “shall” puder ser substituído por “has the duty to”. Por exemplo: X shall (has the duty to) pay Y an amount equal to... The meeting will take place at...
MAY: dever/obrigação de agir; direito/privilégio/poder
MUST: dever/obrigação de agir
MAY NOT: direito/privilégio/poder restrito