The first problem I've encountered is more complicated and I don't have energy to figure it out. The problem was I cannot install the language pack again. Actually when I was fighting against another problem (which I will talk about later) I tried to follow other guides on the Web and removed the en-us folder in Register Editor as shown in Fig.1.
|Figure 1. I've removed en-US at least twice...|
The another problem was that the Date and Time showed an unexpected language (later I found it was Dutch/Belgium).
|Figure 2. Unexpected display language in the Time and Date.|
|Figure 3. gVim|
|Figure 4. gVim|
|Figure 5. Calculator with unfamiliar decimal mark.|
|Figure 6. Excel with dot and comma exchanged.|
|Figure 7. Windows Explorer with unfamiliar decimal mark.|
Okay, the decimal mark was not a big problem but it still annoyed me. The solution was so simple but I didn't know why I could not find the setting for such a long time. I found the Region and Language panel and with unknown reason the format had been set as ``Dutch (Belgium)'' as shown in Fig. 8. Set it back to English, reboot the system, and everything went back to normal state (so far).
|Figure 8. Format can be set in Region and Language.|
 Vistalizator -- Easy way to change display language in Win7
 Decimal mark (from Wikipedia)