Research on inner speech is still in a nascent stage. The present paper investigates consecutive multilinguals' self-reported language use for inner speech with a focus on the L1 (first language) and L2 (second language). Its aim is to identify influential variables in these processes and to investigate if findings from previous studies are applicable across different L1s. The topic is approached using a triangulation approach, which consists of 24 in-depth interviews and a web survey study completed by 167 multilinguals from German-speaking and Asian backgrounds. Quantitative analysis showed that the L1 is generally used more frequently than the L2 in inner speech. Furthermore, high frequency of using the L2, naturalistic exposure to it and high self-reported proficiency in the L2 as well as a high bilingualism index boost its use for inner speech, as does the overall number of languages known. The qualitative analysis furthermore supports the crucial role of these variables in the internalisation of L2 inner speech and shows that they can even shift language use for inner speech entirely from the L1 to English (L2) in case it is frequently and proficiently used and in case a bi-/multilingual experiences naturalistic exposure to English in the L2 environment.