I-526 Petitions may be delayed due to the complexity of the source of funds or project structure, or just the USCIS backlog. Denials can be for the project documentation or issues with the authentication of source of funds. Applications for the green card (after I-526 approval) could be for issues specific to the applicant (such as criminal background or unauthorized entries into the U.S.). Knowing English is not required at this time, but would be for citizenship.
EB-5 petitions are delayed because of not enough experienced USCIS adjudicators processing the petitions. The common reasons for rejection is that the petitions and the supporting documentation are not compliant with the USCIS regulations and policies. To become an EB-5 conditional permanent resident you do not have to know English.