Right-to-left bootstrap multiselect demo (campare with plain input):

شاعران پارسی

فردوسی نظامی گنجوی ابو عبد الله رودکی عُمَر خَیّام نیشابوری نظامی گنجوی خواجه شمس‌الدین محمد حافظ شیرازی ابومحمد مُصلِح‌الدین بن عَبدُالله سعدی شیرازی

درست

ضروری

زبان فارسی

درست

ضروری

On BS inline form:

---

Notes:

(1) Add dir="rtl" into DOM chierarchy, on any ancestor, e.g.:

<form dir="rtl">...</form>

(2) Add rtl support to bootsrap; most probably if you use BS with RTL this step is allready done if not this patch is enough for BsMultiSelect:

<style>
    [dir="rtl"] .custom-control.custom-checkbox{
        margin-left: 0;
        margin-right: 1.5rem;
    }
    [dir="rtl"] .custom-control.custom-checkbox .custom-control-label::after,
    [dir="rtl"] .custom-control.custom-checkbox .custom-control-label::before{
        left: auto;
        right: -1.5rem;
    }
    .was-validated .form-control:valid, .form-control.is-valid{
        padding-right: 0.75rem;
        padding-left: calc(1.5em + 0.75rem);
        background-position: left calc(0.375em + 0.1875rem) center;
    }    
</style>

(3) Patch the BS4 .was-validated problem - there are no way to exclude BS4 styling for concreate `.was-validated :valid` or `.was-validated :invalid` control:

<style>
    .was-validated .dashboardcode-bsmultiselect .custom-control-input:valid:checked ~ .custom-control-label::before {
        border-color: var(--primary);
        background-color: var(--primary);
    }
    .was-validated .dashboardcode-bsmultiselect .custom-control-input:valid:not(:checked) ~ .custom-control-label::before {
        border-color: #adb5bd 
    } 
</style>