I have tried a little bit to find a solution however nothing came out. So I decided to code for myself and put it into my toolbox. The program below lets you change WS default login language. After execution it opens sicf transaction by selecting the node you entered. Deactivate and activate node. It's done.
*&---------------------------------------------------------------------*
*& Report ZBCP_CHANGE_WS_LANGU
*&---------------------------------------------------------------------*
REPORT zbcp_change_ws_langu.
TABLES : icfservice.
DATA : gt_nodes TYPE TABLE OF icfservice.
PARAMETERS : p_lng_fr TYPE sy-langu OBLIGATORY DEFAULT 'EN'.
PARAMETERS : p_lng_to TYPE sy-langu OBLIGATORY DEFAULT sy-langu.
PARAMETERS : p_node TYPE icfservice-icf_name OBLIGATORY.
START-OF-SELECTION.
SELECT * FROM icfservice INTO TABLE gt_nodes
WHERE icf_name = p_node
AND icf_langu = p_lng_fr.
IF sy-subrc NE 0.
MESSAGE 'Node does not exists!' TYPE 'I'.
EXIT.
ENDIF.
LOOP AT gt_nodes INTO icfservice.
icfservice-icf_langu = p_lng_to.
MODIFY icfservice.
ENDLOOP.
MESSAGE 'It''s done please reactivate node !'
TYPE 'S'.
SUBMIT rsicftree WITH icf_serv = p_node AND RETURN.