actually i think the 'GIMP-method' will work on lmms too
In the instakllation there is a folder named 'locale'
Here all the languages that lmms is meant to use lies in .qm files.
The trick is to duplicate the folder with the preferred language, say english
rename the file with the unwanted language to like OLD.._.qm
Now take the copy of the wanted language and rename it to the unwanted one -> qt_en_qm
-to say 'spanis' -> qt_es.qm
Now when lmms start it will load the region specific file (spanish) but the
content of this file will infact be english!
This works perfectly on GIMP. i do not know if you can swing it with lmms, but give it a try :p
Report back if it works