There are 14 languages available for the TCG right now: English, Japanese, Dutch, German, French, Italian, Spanish, Portuguese, Korean, Chinese, Polish, Russian, Thai, Indonesian.
Looking at the HP-translation is indeed a good indication for the Base Set, but unfortunately they’ve changed it a couple of times for the Italian and Spanish cards in other sets. For the ones which are unique and the same across all sets in that language:
- IP is always Dutch
- KP is always German
- PŻ is always Polish
- ОЖ is always Russian
Some of the other languages are also always the same, but unfortunately not unique:
- English always has HP
- Japanese always has HP
- French always has PV
- Portuguese always has PS
- Korean always has HP
- Chinese always has HP
- Thai always has HP
- Indonesian always has HP
As for Spanish and Italian, here are some example sets of how their HP-translations changes in sets:
- Base Set: Italian/French = PV; Spanish = PI; Portuguese = PS
- HeartGold & SoulSilver: French/Italian/Spanish = PV; Portuguese = PS
- XY: French = PV; Italian/Spanish/Portuguese = PS
- etc.
One indication I use a lot with French/Italian/Spanish/Portuguese cards is looking at certain words/sentences. Especially the translation of the phrase “Flip a coin.” is a good indication I use 40% of the time for determining a language of a card for these four:
- French: “Lancez une pièce.”
- Italian: “Lancia una moneta.”
- Spanish: “Lanza 1 moneda.”
- Portuguese: “Lance uma moeda.” / “Jogue uma moeda.”
So when you see one of the attacks starting with “Lancez” it’s French; starting with “Lancia” is Italian; starting with “Lanza” is Spanish; and starting with “Lance”/“Jogue” is Portuguese.
Another thing to look at (with Pokémon, so doesn’t work for Trainer cards and such), is the translation for “weakness”:
- French: “Faiblesse”
- Italian: “debolezza”
- Spanish: “debillidad”
- Portuguese: “Fraqueza”
This one is fail-proof across all sets if I’m not mistaken, but since it’s rather small, I usually look for the “Flip (a coin)” first.
As for Trainers it’s a bit more tricky. After collecting all languages for 3.5+ years I’ve been able to recognize most languages, but even now I sometimes am not sure whether a Trainer card is in Italian, Spanish, or Portuguese (especially Spanish and Portuguese are rather similar). In that case I use the detect language feature of Google Translate, and enter the entire description of the Trainer card (not just a few words, since loads of words are the same in Spanish/Portuguese).
For Energy cards from the Base Set I coincidentally enough made a post yesterday. Here a link to that. All Base Set Energies can be differentiated from one another, except for Chinese/Korean 1st editions, and Chinese/Korean/English UK 4th print unlimited editions. This only applies to the Base Set however.
Also, one small note about the Energy cards in other set: although the “Energy” translation can also for almost any other set across all generations (for which Italian vs Portuguese would be the same in newer sets), there is one exception I know of: German Energy cards from the Expedition Base Set are still “Energy” instead of “Energie”. But, because they do lack the E-reader symbol in the bottom-left, they can still be differentiated from the English versions. I’m however not sure about the Expedition Base Set Energy cards in the French/Italian print runs (will investigate this soon).
As for English vs Japanese vs Korean vs Chinese vs Thai vs Indonesian, I personally immediately recognize the language. Only Japanese and Chinese can sometimes be very slightly difficult, because Japan uses Chinese characters in their alphabet. But in that case the back of the card is a good indication, since Japanese cards will always have either this old back or this new back. All other languages (with a few exceptions like Ancient Mew, English Trainer Decks A/B, one Korean Pikachu promos having a Japanese back, etc.) will have the same back as the English cards.
PS: Here a picture of one Pikachu in every language from my personal collection. 
If with all this information you’re still unsure about a card in particular, feel free to contact me anytime. Hope this all helps.
There are three miniature cards (Turtwig, Chimchar, Piplup) from the 7-Eleven PokéBall Lottery campaign. As you can see in the bottom of this picture, they are smaller than regular TCG cards:
