sorry for not giving more details in the previous post, I wanted to try a couple of additional tests:
1) If I introduce any text (i.e. aaaa) for this string, after saving and editing the string is empty in WTS GUI. I downloaded the translated file (ac-btf_str.properties) and checked that the string was still in English in the file.
2) I edited the file in my PC, modified the string, loaded the file in the extension XPI file and tested in Firefox: OK. Then uploaded ac-btf_str.properties to WTS, edited and ... the string was empty in the WTS Spanish GUI (in order to make sure, I downloaded the file and checked that the string was still in English).
3) I deleted the variable and the translation in the file in my PC and typed them again (just in case some hidden character or similar). Tested in Firefox: OK, uploaded in WTS: same result that case 2.
4) I downloaded the en-US ac-btf_str.properties, and then uploaded it in WTS (like Spanish file). I edited it and saw in the WTS GUI the string in English but not empty (I thought: good, we are improving). Then translated to Spanish all the strings and saved. When editing again, the string was empty (in WTS GUI). Uploaded then the real Spanish locale from my PC and found that now according to WTS I have 6 (instead of 5) strings in the file, although I can only see 5. I downloaded the file and checked in my PC: only 5 strings. And now every time I try to modify the string, the English string appears again in WTS GUI (instead of empty string like before).
Two (possible) clues:
BMB extension has the same variable with the same text, I have translated it in WTS without any problem.
I have checked Polish locale for this extension (AB&HF) currently in progress (97%). There is only a string not translated yet in WTS: ac-btf.settings.loaderr in ac-btf_str.properties!
I hope my explanation even if a bit long, will be understandable.
After this post I made a try: extracted again en-US ac-btf_str.properties file from original XPI, translated it to Spanish and uploaded in WTS. Editing the file in WTS I could see in WTS GUI variable ac-btf.settings.loaderr twice: in Spanish and in English. Then I deleted English string, saved and edited again: Now I can see in WTS GUI ac-btf.settings.loaderr three times: in English, Spanish and empty, you can see it by yourself (I do not find any smilie able to express my feelings). At this moment I decided not to try any further test by myself because I am afraid I will provoke some disaster. I am looking forward to new instructions.