feat: add --language flag for non-English TTS voices (Dutch support)

- Add LANGUAGE default (en) to shell.functions
- setup_python_venv downloads Dutch ONNX voices (pim, ronnie, nathalie)
- wake_word_sample_generator uses multiple --model flags for single-speaker
  voices, cycling between them for variety
- train_wake_word accepts and passes --language through the pipeline
- recorder_server.py accepts language in session API
- Web UI adds language dropdown (English/Dutch)

cli/shell.functions

@@ -11,12 +11,14 @@ fi
 DEFAULT_SAMPLES=50000
 DEFAULT_BATCH_SIZE=100
 DEFAULT_TRAINING_STEPS=40000
+DEFAULT_LANGUAGE=en
 
 [ -f "${DATA_DIR}/.defaults.env" ] && source "${DATA_DIR}/.defaults.env" || :
 
 : "${SAMPLES:=${DEFAULT_SAMPLES}}"
 : "${BATCH_SIZE:=${DEFAULT_BATCH_SIZE}}"
 : "${TRAINING_STEPS:=${DEFAULT_TRAINING_STEPS}}"
+: "${LANGUAGE:=${DEFAULT_LANGUAGE}}"
 : "${CLEANUP_WORK_DIR:=false}"
 : "${CLEANUP_ARCHIVES:=false}"
 : "${CLEANUP_INTERMEDIATE_FILES:=false}"