fix(grid,flex): fix compile errors when grid or flex is disabled (#5289)

demos/multilang/lv_demo_multilang.h

@@ -15,6 +15,16 @@ extern "C" {
  *********************/
 #include "../lv_demos.h"
 
+#if LV_USE_DEMO_MULTILANG
+
+#if LV_USE_GRID == 0
+#error "LV_USE_GRID needs to be enabled"
+#endif
+
+#if LV_USE_FLEX == 0
+#error "LV_USE_FLEX needs to be enabled"
+#endif
+
 /*********************
  *      DEFINES
  *********************/
@@ -32,6 +42,8 @@ void lv_demo_multilang(void);
  *      MACROS
  **********************/
 
+#endif /*LV_USE_DEMO_MULTILANG*/
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif