In InitializeComponent() method (in .designer.cs file), add:
txtCL_VL_LA.Enter += SetLAO;
txtCL_FN_LA.Enter += SetLAO;
txtCL_LN_LA.Enter += SetLAO;
txtCL_VL_LA.Leave += SetENG;
txtCL_FN_LA.Leave += SetENG;
txtCL_LN_LA.Leave += SetENG;
You may have to be careful as to where you put these lines in the code file. Observe the structure of this file and decide where to put each line.