The realm of New Englishes offers enriching avenues to explore the interplay between language acquisition and sociolinguistic influences in linguistically diverse ecologies. Yet research into this interdisciplinary arena remains lacking. Accordingly, this thesis addresses this paradigm gap by focusing on the Malaysian ecology. One of the three empirical studies conducted as part of this project is i) a CASE STUDY which examines the morphosyntactic properties of an indigenised variety of English viz., Colloquial Malaysian English (CME). The data generated from naturalistic conversations came from two pairs of adult Malaysians with different L1 backgrounds (i.e., Malay and Chinese). While many of the non-standard features supplied could be explained by substrate influence, there were also features resembling general second language (L2) behaviours and creative innovation. The MAIN STUDY adopts a concurrent embedded design, which comprises ii) an ACQUISITIONAL STUDY and iii) a SOCIOLINGUISTIC STUDY. The ACQUISITIONAL STUDY investigates the roles of the first language (L1) and CME in the ultimate acquisition of finiteness in Standard English (StE). The adult participants recruited for this study were 145 Malaysians and 30 British (control). Malaysians who acquired English as (one of) their L1(s) (L1-MalE(+)) were predicted to have less difficulty than their L1-Malay and L1-Chinese peers and perform more similarly to the British English (BritE) monolinguals. This is because, despite the prevalence of CME in the local environment, L1-MalE(+) learners would merely have to reset the optional features of finiteness in CME to obligatory, as required in StE. Meanwhile, L1-Malay and L1-Chinese learners would be faced with an additional learnability burden of acquiring finiteness as a new functional feature, given its absence in their L1s. Findings from a grammaticality judgement task and narrative task revealed that although the Malaysian cohort behaved statistically differently from the L1-BritE control, the L1-MalE(+) groups outperformed the L1-Chinese and L1-Malay groups across the board. That said, the L1-Malay group fared considerably better than its L1-Chinese counterpart and was about on par with the L1-MalE(+) peers. These findings indicated clear L1 effects modulated by typological proximity. Meanwhile, the SOCIOLINGUISTIC STUDY explores Malaysians’ attitudinal behaviours towards CME and StE. The same participants from the acquisitional study undertook a sociolinguistic survey administered for this study. Findings revealed that the participants were non-discriminatory towards CME and StE, and that they were aware of when to use these varieties across different social settings. Altogether, this thesis demonstrates the facilitative role of CME in the acquisition of StE, and concurrently vindicates the functional importance of CME and StE as legitimate varieties in the Malaysian milieu.