Measures of language proficiency and dominance have become increasingly important in the study of bilingualism. Yet, there is still no agreed-upon measure of language proficiency and dominance. A critical challenge is isolating language abilities from cognitive control abilities and from subjectivity related to introspective processes. We introduce a new method of measuring bilinguals’ language proficiency and language dominance based on the sustainability hypothesis, which has been used to measure perceived task difficulty (Rosenbaum & Bui, 2019). According to the sustainability hypothesis, when assessing task difficulty, individuals may retrieve information about how well they think they can do the tasks repeatedly such that task difficulty is inversely related to task sustainability. We applied this concept to the study of language dominance in bilingualism. Mandarin-English bilinguals (n = 120) completed a picture naming task in Mandarin and English and then answered 2-alternative forced choice questions about which of two tasks seemed easier: naming some number of pictures, n(M), in Mandarin or some number of pictures, n(E), in English. The same participants also indicated which of the two tasks they would prefer to do first. By pairing the values of n(M) and n(E) in all possible ways, we calculated the point of subjective equality (PSE) for the difficulty of naming pictures in Mandarin or English and for the preferred ordering of the two languages. The PSEs for both decisions were closely related to individual differences in language proficiency as measured by picture naming accuracy, verbal fluency, and self-report. Including both an objective language assessment score and a subjective evaluation of difficulty holds promise for assessing language proficiency and dominance in ways that do not rely on introspection or cognitive control.