This is a borderline case of 'possible non-reality'. In the more obvious case of 'busco unos zapatos que sean cómodos' the subjunctive signals that such shoes might not exist. Here we could more comfortably use the indicative 'que viven' on the assumption that there actually are foreigners living in Japan, the subjunctive is a pedantic touch here, making no such assumption. Juan Carlos is correct to point out possible future foreign residents as an extension of the possible present ones covered by the above.

Since it refers to a condition to be met, the use of subjunctive is necessary. This will often be the case when talking about law or something that has to do with requirements. Take this example: Los motociclistas que no utilicen el casco tendrán una multa de un salario mínimo. Hope this helps.

Este uso del presente de subjuntivo es para expresar una idea virtual sobre el futuro. Si lo cambiamos a presente de indicativo cambia el signficado y se referiría a todos los habitantes actuales de Japón. Ej: Las medidas, presentadas el lunes por el Ministerio de Justicia, cubrirán a todos los ciudadanos extranjeros que viven en Japón, incluyendo estudiantes.

This an advise sentence, or and futur idea