Currently, more than half of the world's population speaks more than 1 language. As a consequence, psychotherapists are more and more likely to encounter clients whose first language is neither their own nor the first language of the community they live in. This language diversity, which is situated at the intersection of linguistics, cognitive, social, and cultural psychology, anthropology, and second language acquisition, has the potential to significantly impact the practice and success of clinical psychological interventions. It is surprising, therefore, that research on multilingualism in psychotherapy is rare.
Therapy is relational, and language is a crucial part of all human relationships [Espín, 2013]. Multilingualism, defined as using 2 or more languages, affects individuals in several domains. The available evidence suggests that multilinguals tend to differ from monolinguals in their linguistic and cognitive organization, which affects the recall of personal experience, perception, comprehension, memory, and self-perception. Language also has important effects on the development of identity. In a range of studies, participants reported feeling different when using another language, which gave rise to the concept of ‘language selves'. There is also evidence for increased problem-solving skills, flexibility in thinking, and better executive functioning in multilinguals. Finally, there have been interesting findings about changes in attentional processing, judgment, and decision making of multilinguals - functions that may have an impact on symptom expression [Foster, 2001)] and the process of psychotherapy.
Research on multilinguals' use of language has spawned a debate on the effectiveness of psychotherapy treatment when the therapist and the client are linguistically diverse. Multilingualism clearly has important implications for the process of psychotherapy. Bradford and Muñoz  concluded that the degree of the therapist's familiarity with the client's language correlates with diagnosis, assessment of symptoms, and the perception of mental illness. After reviewing psychosocial, psychotherapeutic, and neuropsychological evidence on the interaction of phenomena associated with memory, cognition, and emotion in bilingual therapy, de Zulueta  stated that, ideally, both therapists and clients should engage in the therapeutic process in their respective first language. The use of a client's native language in therapy can help the client to feel more comfortable and to perceive the therapeutic context as more meaningful. When individuals can speak passionately in their first language, this can serve as a powerful emotional release [Espín, 2013]. In addition, language-concordant therapists can understand cultural nuances that might go unnoticed by a therapist who does not speak the patient's language [Bloom et al., 2005].
Receiving and providing therapy in another than the mother tongue has been examined in relation to code switching, which is defined as changing from one language to another during the course of conversation [Wong et al., 2016]. The emotional distancing linked to the use of the second language may facilitate the verbalization of highly charged material. Even though the dominant language has richer emotional structures, which can capture a greater richness of experience, the emotional potency of the native language may impede cognitive processes and, therefore, access to intellectual resources for making sense of experiences. Where there is a possibility of code switching, this option may be strategically used in therapy.
The richness and variety of information representing multilingual memory may be accessible through the use of language nuances and variables that structure communication differentially in several languages. The use of different languages might reveal more varied information than the use of just 1 [Altarriba, 2002]. Multilingualism potentially confers several benefits to the process of psychotherapy. For instance, clients can be more expressive since they are not limited to just 1 language. The client has a choice as to which language to use and is thereby able to select the word that most clearly captures the essence of what they are trying to communicate.
Therapists can develop a capacity to withstand, understand, and creatively use the complexities of their linguistic experience in multilingual therapy - a supporting point for the notion that therapists' multilingualism can enhance and bring unique advantages to the therapeutic process. Multilingualism enables therapists to follow clients not only across languages but also through multiple areas of associations and meanings. Bilingual therapists possess greater flexibility in the usage of language in psychotherapy than monolingual therapists [Burck, 2004]. These cognitive abilities can help clinicians to work in new ways, absorb new perspectives, and process the large amount of stimuli they encounter when interacting with clients from different cultures.
There is growing recognition that multilingualism of therapists and clients has important and far-reaching implications on psychotherapy. This becomes apparent in the increasingly diverse topics, which are currently being investigated within the field. The research on multilinguals' use of language has sparked the debate on the effectiveness of psychotherapy treatment, diagnosis, and therapeutic relation when the therapist and the client are linguistically diverse.
Psychotherapy is best conducted in the stronger language in order to tap into internal cognitive processes [Cofresi and Gorman, 2004]. The assessment should be conducted in the language most compatible with the multilingual client's language proficiency and dominance. Given that client's abilities to express emotional and psychological experiences may vary widely across their languages, treatment may need to be bilingual in order to benefit from the strengths of both linguistic systems. Using only 1 language in therapy may block access to some mental conflicts and affective processes [Marrero et al., 2002].
Further research in this area should explore specific aspects such as the effects of multilingualism on the outcome and effectiveness of psychotherapy. Moreover, further research on language switching in therapy, how it is initiated, and its characteristics would be important.