Summary

msls_get_switcher() is the documented public template tag for rendering the
language switcher from theme code. Its signature requires a positional
argument, but the body already treats that argument as optional — any
non-array value is coerced to an empty array. As a result, the natural no-arg
call msls_get_switcher() throws a fatal ArgumentCountError even though it
would otherwise work fine.

Current behavior

includes/api.php (as of 3.0.0):

function msls_get_switcher( $attr ): string {
    $arr = is_array( $attr ) ? $attr : array();   // non-array already → empty array
    $obj = apply_filters( 'msls_get_output', null );

    return ! is_null( $obj ) ? strval( $obj->set_tags( $arr ) ) : '';
}

Calling it without arguments:

echo msls_get_switcher();

fatals the page with:

ArgumentCountError: Too few arguments to function msls_get_switcher(),
0 passed and exactly 1 expected

Expected behavior

The no-arg call should work. Theme authors reasonably expect it to, because:

• the body already normalizes anything non-array to array(), so $attr is
  effectively optional;
• the sibling template tag msls_the_switcher( array $arr = array() ) already
  defaults its argument.

Use case

A theme rendering the switcher with default markup just wants:

echo msls_get_switcher();

Today it has to pass msls_get_switcher( array() ) purely to satisfy the
signature.

Proposed change

Give the parameter a default, matching msls_the_switcher():

function msls_get_switcher( $attr = array() ): string {

The parameter is intentionally left untyped so the existing is_array()
coercion keeps working. This preserves the [sc_msls] shortcode
(add_shortcode( 'sc_msls', 'msls_get_switcher' )), which WordPress invokes
with the empty string '' when no attributes are given — an array type hint
would break that case. Existing msls_get_switcher( $attr ) callers and the
deprecated get_the_msls( $attr ) shim are unaffected.

Backward compatible; no behavior change for any current caller.