@Sudsy
English VS Japanese
Pretending to not speak English, when they can, is a very popular and ridiculously insane game played by many Japanese women.
1. Many Japanese are embarrassed if they can't speak English perfectly.
That is, they can speak English and have YEARS of study, but don't want to make any mistakes.
The foreigner has to then encourage the Japanese person to use their English and tell them NOT TO WORRY about mistakes, as oppose to the situation in other countries, where people are enthusiastic to practice or be taught English (even if they only know a little and make mistakes). This also goes into the Japanese cultural preference for sneakiness.
2. Various Japanese guys and xenophobic groups (to include other Japanese women) can label Japanese women that openly speak English well as "sluts".
This is of course absolutely ridiculous and creates a conflict that makes some Japanese women very double-minded and sneaky about their English ability.
They may practice English for jobs, travel, etc... But are afraid of getting "slut" labels by certain Japanese. So they can be very wary of OTHER Japanese that might be WATCHING them.
Again, the foreigner will at times have to encourage and convince the Japanese woman that it's alright to use her English OR she is in a location where she feels safe to speak English. For example, a language exchange party or club with lots of foreigners.
And certain xenophobic Japanese can give Japanese women the "slut" label, for just associating with foreigners, irregardless of if you are speaking Japanese or English. But magically, no matter how many Japanese guys she talks to or how many foreign women the Japanese guy talks to, it's fine.
3. My pals that talk directly to ladies in the city and who are fluent in Japanese, will still often start conversations in English.
That is, we START conversations in English to guage the language ability, her past interaction with foreigners, and how xenophobic she might be.
Did she live in a foreign country? Was she an exchange student? Does she like to travel? Does she like aspects of foreign culture?
Japanese that live in a bubble world with little to no interaction with foreigners and no interest in the world outside Japan, tend to not be worth investing time in. That is, they can be way too paranoid, get weirdly hysterical, and have excessive negative views of foreign people. Not always, but too often that's the case.
4. Certain Japanese label foreigners that speak Japanese "too well" as "playboys".
This is often from the same sources and ignorant groups that label Japanese women that speak English well as "sluts". Also keep in mind that the theme is to attack foreign male and Japanese female couples. And sadly, foreign females may also join in on such attacks out of jealous and envy.
If a foreign man, outside of the job context, starts a conversation in Japanese there is a negative stereotype attached that he is a playboy with several Japanese girlfriends. This is again, asinine and ridiculous beliefs among ignorant Japanese, but they are annoyingly popular and perpetuated by jealous or anti-foreigner Japanese guys/groups. It's a way to knock down foreign guys, BEHIND THEIR BACKS, who appear to be doing well.
A way to deal and avoid this negative stereotype, is to start conversations off in English, then to add Japanese as necessary to be better understood. You can encourage the Japanese person to practice their English and you can request that they help you practice Japanese. This makes the interaction, especially when you are strangers, "feel" more acceptable and "innocently accidental".
Ways Of Using Both English & Japanese
If her English is truly horrible (and after you gauged her ability for a bit), then you would interject more and more Japanese into the conversation, if you can.
And I'm not joking about the odd Japanese female tendency towards hysteria. I've primarily only encountered this level of "craziness" in Japan. A Japanese woman can have been so much into an isolated bubble and have so many odd conflicting thoughts and fears about foreigners, that speaking to a foreigner or even hugging/touching can send a few into a panic. A foreign man, will need to be conscious of Japanese cultural weirdness, if he wants to be successful with pickup and not be confined to only P4P or job related "meeting parties".
Lots of Japanese want to communicate with people from other countries and cultures, but have massive fears, phobias, and social anxiety. If a foreigner understands this, both English and Japanese can be used. Many will want to practice English, and many will be comfortable using only Japanese, and there will be a lot in the middle.