L1 is TEFL jargon: first language.
If you train in let's say England your students for teaching practice are likely to come from several different countries. Potentially you could have 12 students in a class, each from a different country/language. It is therefore virtually impossible for you to be tempted into using their first language, and everything must be explained, instructed, conversed in English. If you train in let's say France and you happen to speak French there might be a temptation for you to sometimes use French (L1) as a kind of shorthand - no doubt great for your French, but nobody learns English by speaking French.
If you can teach English to 12 students who speak 12 different languages you can teach it to Martians. IMHO, if there is ANY advantage to training in the country where you plan (first) to teach it is very localized and extremely short-lived.
I'm not saying that you absolutely must train in an English-speaking country, just that you shouldn't allow yourself to be hoodwinked by a half-baked idea :)
Btw, you'll find L1 and other jargon in ELT Terminology