How can I change the language (to english) in Oracle SQL Developer?

Try to set user.language and user.country properties in sqldeveloper.conf file located in sqldeveloper\bin folder (%APPDATA%\sqldeveloper\<version>\product.conf for more recent versions) .
Sample for set french language:

IncludeConfFile ../../ide/bin/ide.conf

SetJavaHome D:\jdk1.6.0_25

AddVMOption  -Doracle.ide.util.AddinPolicyUtils.OVERRIDE_FLAG=true

AddVMOption -Dsun.java2d.ddoffscreen=false

AddVMOption -Dwindows.shell.font.languages=
AddVMOption -Duser.language=fr
AddVMOption -Duser.country=FR
AddVMOption  -XX:MaxPermSize=128M

AddVMOption -Doracle.jdbc.mapDateToTimestamp=false  
IncludeConfFile  sqldeveloper-nondebug.conf

To switch off language try to remove its support by editing oracle.translated.locales option in ide.boot file (located in ide\bin folder).
For example, if I want to 'break' french language support this option will be:

oracle.translated.locales = de,es,it,ja,ko,pt_BR,zh_CN,zh_TW

The original value was:

oracle.translated.locales = de,fr,es,it,ja,ko,pt_BR,zh_CN,zh_TW

The configuration above (sqldeveloper.conf file) will be wrong after remove french support and the french language does not appear as GUI localization.

You can also set language at runtime

sqldeveloper.exe --AddVMOption=-Duser.language=en

to avoid editing sqldeveloper.conf every time you install new version.

You can also configure directly on the file ..sqldeveloper\ide\bin\ide.conf:

Just add the JVM Option:

AddVMOption -Duser.language=en

The file will be like this: